Tutors Answer Your Questions about Sequences-and-series (FREE)
Question 571989: IN A PILE OF LOGS, EACH LAYER CONTAINS ONE MORE LOG THAN THE LAYER ABOVE,IN THE TOP LAYER CONTAINS JUST ONE LOG. IF THERE ARE 105 LOGS IN THE PILE, HOW MANY LAYERS ARE THERE? Answer by htmentor(581) (Show Source):
You can put this solution on YOUR website!IN A PILE OF LOGS, EACH LAYER CONTAINS ONE MORE LOG THAN THE LAYER ABOVE,IN THE TOP LAYER CONTAINS JUST ONE LOG. IF THERE ARE 105 LOGS IN THE PILE, HOW MANY LAYERS ARE THERE?
======================
This is an arithmetic sequence: 1,2,3,4... [counting down from the top of the pile]
The nth term of an arithmetic sequence is
a(n) = a(1) + (n-1)d where a(1) = the 1st term, d = the common difference
In this case a(n) = 1 + (n-1) = n
The sum of the 1st n terms of the sequence is S(n) = (n/2)(a(1) + a(n)) = 105
So S(n) = (n/2)(1+n) = 105
Solve for n:
n^2 + n - 210 = 0
Factor:
(n-14)(n+15) = 0
Take the positive solution, n=14
So there are 14 layers
Question 571213: geometric mean between 99 and 44 Found 2 solutions by Theo, mrjunecarlo1095@ymail.com: Answer by Theo(2967) (Show Source):
You can put this solution on YOUR website!the geometric mean of a series of numbers is found by multiplying all the numbers together and then taking the nth root of the product.
n is the total number of numbers in the series.
in your problem, n is equal to 2.
you multiply the numbers together to get 4356 and then take the square root of that to get 66.
Answer by mrjunecarlo1095@ymail.com(7) (Show Source):
You can put this solution on YOUR website! The square root of the product of the two numbers is the geometric mean,
So having the product of 99 and 44 is 4356,
get the square root of 4356,
It's 66.
The geometric mean between two numbers is 66.
Question 571200: what is the 11th term of the geometric sequence 81,-27,9? Answer by Theo(2967) (Show Source):
You can put this solution on YOUR website!looks like the common ratio is -1/3.
81 * -1/3 = -27 * -1/3 = 9, etc.
An = A1*r^(n-1) i believe.
first term is A1 which is equal to 81.
second term is 81 * -(1/3)^1 = -27
third term is 81 * -(1/3)^2 = 9
the 11th terms should be:
81 * -(1/3)^10 = .001371742
as a fraction it would be equal to:
81 * -(1/3)^10 which is equal to:
81 * (-1)^10 / 3^10 which is equal to:
81 * 1 / 59049 which is equal to:
81 / 59049
the decimal equivalent of 81 /59049 is equal to .001371742.
if you were to explicitly analyze the sequence, you would get the following table.
sequence number number
1 81
2 -27
3 9
4 -3
5 1
6 -1/3
7 1/9
8 -1/27
9 1/81
10 -1/243
11 1/729
note that 1/729 is equivalent to 81 / 59049
multiply 1/729 by 81 / 81 and you get 81 / 59049
Question 570772: In the figure below, line m is parallel to line n, and line t is a transversal crossing
both m and n. Which of the following lists has 3 angles that are all equal in measure?
A. ∠a, ∠b, ∠d
B. ∠a, ∠c, ∠d
C. ∠a, ∠c, ∠e
D. ∠b, ∠c, ∠d
E. ∠b, ∠c, ∠e Answer by richard1234(4796) (Show Source):
Question 570818:
2. Which number should come next in the series?
1,4,9,16,25
27
34
36
45
I don't know Answer by KMST(592) (Show Source):
Question 570216: The sum of x and 5 is 16. Answer by jim_thompson5910(21667) (Show Source):
Question 570125: I am having issues with determining a of n when the only given are two terms from an arithmetic series. The two terms given are a of 8= 75 and a of 20= 75. What would a of n be?? Answer by htmentor(581) (Show Source):
You can put this solution on YOUR website!I am having issues with determining a of n when the only given are two terms from an arithmetic series. The two terms given are a of 8= 75 and a of 20= 75. What would a of n be??
========================
The nth term of an arithmetic series can be written
a(n) = a + (n-1)d where a=the 1st term, d=the common difference
The information provided gives us 2 equations and 2 unknowns:
a(8) = 75 = a + 7d
a(20) = 75 = a + 19d
If both a(8) = 75 and a(20) = 75, then the common difference would be 0, which isn't much of a sequence. You should check the problem again.
Question 569333: the equation is 1+2+4+8..., n-6 i know the answer is 63 but idk how to get the answer and what formulas to use Answer by solver91311(12126) (Show Source):
Question 568967: write the first three terms of an arithmetic sequence if the fourth term is 10 and d=-3. Answer by scott8148(5880) (Show Source):
Question 568427: How many terms are there in this sequence: 12,15,18,21.......267 Answer by richard1234(4796) (Show Source):
You can put this solution on YOUR website!Subtract 9 from each term in the sequence.
3, 6, 9, ..., 258
Divide each term in the sequence by 3.
1, 2, 3, ..., 86, so there are 86 terms in the sequence. The operations we did (subtract 9, divide by 3) make it easier to count how many terms are in the sequence, without changing the number of terms in the sequence itself.
Question 568527: not getting the understanding of these problems and they need to be self explanatory to me.
8,11,14,17,20
1,16,81,256,625
5,15,45,135,405
2,7,12,17,22
1,1/2,1/4,1/8,1/16 Answer by jim_thompson5910(21667) (Show Source):
You can put this solution on YOUR website!Hints:
8,11,14,17,20 -- adding 3 each time
1,16,81,256,625 -- 1 = 1^4, 16 = 2^4, 81 = 3^4, ...
5,15,45,135,405 -- multiplying by 3 each time
2,7,12,17,22 -- adding 9 each time
1,1/2,1/4,1/8,1/16 -- dividing by 2 each time
Question 568401: Please help me in solving this arithmetic sequence.: The 21st term of the arithmetic sequence is -48, and the 33rd term is -84. Find the 101st term. Answer by htmentor(581) (Show Source):
You can put this solution on YOUR website!Please help me in solving this arithmetic sequence.: The 21st term of the arithmetic sequence is -48, and the 33rd term is -84. Find the 101st term.
===============================
The formula for the n-th term of an arithmetic sequence is:
a_n = a_1 + (n-1)d where a_1 = the 1st term, d = the common difference
The information provided gives us two equations in two unknowns:
a_21 = -48 = a_1 + 20d [1]
a_33 = -84 = a_1 + 32d [2]
Subtract [2] from [1]:
36 = -12d
d = -3
Now substitute back into one of the equations above to get a_1:
-48 = a_1 + 20*-3
a_1 = 12
So the formula is a_n = 12 - 3(n-1) = 15 - 3n
Therefore the 101st term is a_101 = 15 - 3*101 = -288
Question 568043: please explain how to find only thee sum of the odd numbers in sigma notation n=1-35 Answer by KMST(592) (Show Source):
You can put this solution on YOUR website!You could say that odd numbers can be written as  .
All odd numbers can be written that way for some natural number  , and the expression  gives you an odd number for every natural number  .
 ,  , ... 
You are trying to find the sum of the first 18 odd numbers:
S=1+3+5+ ... +31+33+35
There are many ways to go about it.
BLAZING YOUR OWN TRAIL (or rediscovering gunpowder):
2S=S+S=(1+3+5+ ... +31+33+35)+(1+3+5+ ... +31+33+35)=1+3+5+ ... +31+33+35+1+3+5+ ... +31+33+35.
Because of commutative and associative properties, we can re-arrange that sum, pairing the first number with the last, second with second last, and so on, to get:
2S=(1+35)+(3+33)+(5+31)+ ... +(31+5)+(33+3)+(35+1)=36+36+36+ ... +36+36+36= 
2S=  --> S=  =324 
INVOKING ARITHMETIC SEQUENCES (if that's what you are studying)
The sum that you want is 
That is the sum of an arithmetic sequence (or arithmetic progression). You may write the terms explicit formula as  , with first term  and common difference  .
(That is equivalent to  ).
Naming conventions may vary, and you may use 
You may have been given the formula:
 or 
You would calculate
 or
knowing  from the start, you may rather calculate

Or maybe your naming conventions and formulas are a bit different.
Anyway, you'll get the answer 
INVOKING KNOWN SUMS
You may have been told that the sum of the fist n natural numbers is
1+2+3+ ...+(n-1)+n= 
Then, the sum of all integers from 1 to 35 is  and can be written as the sum of the odd numbers from 1 to 25 plus the sum of the first 17 even numbers from 2 to 34:

The sum of the first 17 even numbers is twice the sum of the first 17 natural numbers:
(2+ 
So  --> 
Anyway, you'll get the answer
Question 567832: the 15th term in an arithmetic sequence is 43 and the sum of the first 15 terms of the series is 120. Determine the first three terms of the series Found 2 solutions by issacodegard, solver91311: Answer by issacodegard(60) (Show Source):
You can put this solution on YOUR website!Call the terms a_1,a_2,... then S_15=15*(a_1+a_15)/2=120. So, we solve for a_1,
a_1=-27. Then we need to know the common difference, d. We know that a_n=a_1+(n-1)d. So, d=(43-(-27))/14=5. So, a_2=a_1+d=-22, and a_3=a_2+d=-17.
Answer by solver91311(12126) (Show Source):
You can put this solution on YOUR website!
The sum of an arithmetic sequence is given by:
Where is the number of terms, is the first term, is the last term, and is the sum of terms.
Plug in the given values of , , and and then solve for
Once you have the value for , you can use that to find , the common difference because the th term is given by:
Plug in the values you know for , , and , then solve for .
Finally, calculate:
and
John

My calculator said it, I believe it, that settles it
Question 567791: sarah can shovel her driveway in 35 minutes. John can do the same job in 30 mintues. If they work together how many minutes would it take to shovel the driveway.
I got as far at t/30+t/35=1 Answer by Earlsdon(6103) (Show Source):
Question 567777: Suppose that the sum of two whole numbers is 20, and the sum of their reciprocals is 5/16. Find the numbers. Answer by stanbon(48545) (Show Source):
You can put this solution on YOUR website!Suppose that the sum of two whole numbers is 20, and the sum of their reciprocals is 5/16. Find the numbers.
----------------------
Equations:
x + y = 20
1/x + 1/y = 5/16
-----------
x + y = 20
(x+y)/xy = 5/16
-----
20/xy = 5/16
5xy = 320
xy = 64
--
Remember x + y = 20
x = 20-y
---
Substitute:
(20-y)y = 64
20y-y^2 = 64
---------------
y^2-20y+64 = 0
y^2-16y-4y+64 = 0
y(y-16)-4(y-16) = 0
(y-16)(y-4) = 0
---
If y = 4; x = 16
If y = 16; x = 4
=======================
Cheers,
Stan H.
Question 567485: A combination lock for a bicycle contains four digits, each of which can have a value of 0, 1, 2, 3 or 4. How many different sequences of numbers can open this lock? I came up with 120 digit combination. Answer by scott8148(5880) (Show Source):
Question 566625: Given the sequence 1331, 1000, 729, x, y, z, 125, . . . , what is the sum of x, y, z? Please show me the steps. Answer by richard1234(4796) (Show Source):
You can put this solution on YOUR website!1331 = 11^3
1000 = 10^3
729 = 9^3
.
.
.
125 = 5^3
Therefore, x,y,z = 8^3, 7^3, 6^3 or 512, 343, 216. Their sum is 512+343+216 = 1071.
Question 566162: The question is: what is the sum of 4+11+18+....+4001
I have been taught to add the first and last number, so i get 4005. Then I usually divide by half of the last number, which would be 2000.5
However, I know this only works if the numbers go up by one, not seven. This is where I get lost. Answer by richard1234(4796) (Show Source):
You can put this solution on YOUR website!Let S = the sum you want. Then, we have
S = 4 + 11+18+...+4001
S = 4001+3994 + ... + 4
If we add them up, we get 2S = 4005 + 4005 + ... + 4005. Now we have to count the number of "4005s" there are, which is equal to the size of the set {4, 11, 18, ..., 4001}. This is equal to the size of the sets {7, 14, 21, ..., 4004} and {1, 2, 3, ..., 572} (we are just adding or multiplying numbers in the sets by a fixed number; this does not change the size of the set).
Therefore, there are 572 "4005s." Hence, 2S = 572*4005, S = 572*4005/2 = 1145430.
Question 566014: S10=-20 and d=4 find a1
Answer by stanbon(48545) (Show Source):
You can put this solution on YOUR website!S10=-20 and d=4 find a1
-----
S(10) = a(1) + (n-1)d
-20 = a(1) + 9*4
a(1) = -20-36
a(1) = -56
================
Cheers,
Stan H.
==================
Question 565988: write the first three terms of an arithmetic sequence if the fourth term is 10 and d=-3 Answer by Tatiana_Stebko(1060) (Show Source):
Question 565635: Which number should come next at the end of this series: 1/12, 1/6, 1/3, 2/3?
Possible answers: 3/4, 4/3, 3/2 Answer by ad_alta(170) (Show Source):
Question 565548: find the sum of the first 30 terms of 4+7+10+13+16+19.... Answer by htmentor(581) (Show Source):
You can put this solution on YOUR website!find the sum of the first 30 terms of 4+7+10+13+16+19....
==========================
This is an arithmetic sequence with a_1 = 4, and a common difference of 3.
The formula for the n-th term is:
a_n = a_1 + (n-1)d
a_n = 4 + 3(n-1) = 3n + 1
So the 30th term is a_30 = 3*30 + 1 = 91
The sum of the 1st n terms of an arithmetic sequence is:
S_n = (n/2)(a_1 + a_n)
So S_30 = (30/2)(4+91) = 95*15 = 1425
Question 564340: I need to know the formula for adding whole numbers (1+2+3+4...) to get a sum over 1000. I know the answer is 45 but I need the formula to figure it out.
Answer by ad_alta(170) (Show Source):
You can put this solution on YOUR website!I should probably let you discover it on your own (its more fun that way), but the answer is n(n+1)/2. Since we're talking about n=1000, 1000(1001)/2=500,500.
Or in your case, (1+2+3+4+5+6+7+8+9)=(9)(10)/2=45.
Question 563844: find the missing number, n, in data set
28,n,2,19, and 28
mean = 19 Answer by prateekagrawal(47) (Show Source):
Question 563669: Are the sets equal? {4, 4, 11, 11, 17} = {4, 11, 17} Answer by jim_thompson5910(21667) (Show Source):
Question 563150: How can we show if the sequence αn = 2cos(nπ) is monotone or not? Answer by richard1234(4796) (Show Source):
Question 562803: What is the nth term for this pattern?....... 2,8,18,32,50 Found 2 solutions by issacodegard, scott8148: Answer by issacodegard(60) (Show Source):
You can put this solution on YOUR website!Hi, the first term is 2, and I'll denote the nth term by f(n). The sequence of terms for f(n)/2 is 1,4,9,16,25,... which are the positive integers squares. So we have f(n)/2=n^2. Therefore, f(n)=2n^2.
Answer by scott8148(5880) (Show Source):
Question 562232: What is the common difference in the arithmetic sequence of 1, 1/4, 1/9, 1/16? Answer by richard1234(4796) (Show Source):
Question 562209: What is the arithmetic sequence of 1, 1/4, 1/9, 1/16? Answer by richard1234(4796) (Show Source):
Question 561862: The sum of 40 consecutive integers is 100 what is the largest integer
Answer by Edwin McCravy(6936) (Show Source):
You can put this solution on YOUR website!The sum of 40 consecutive integers is 100 what is the largest integer
The largest integer is the 40th term, or a40
Consecutive integers form an arithmetic sequence with a common difference of 1.
This is an aritmetic series with n=40 terms and S40 = 100
The formulas we will use are:
Sn = [2a1 + (n-1)d] and an = a1 + (n-1)d
where n = 40, Sn = S40 = 100, d=1
S40 = [2a1 + (40-1)·1]
100 = 20[2a1 + 39·1]
100 = 20[2a1 + 39]
Divide both sides by 20
5 = 2a1 + 39
-34 = 2a1
-17 = a1
The largest integer is the 40th term a40
an = a1 + (n-1)d
a40 = -17 + (40-1)·1
a40 = -17 + 39·1
a40 = -17 + 39
a40 = 22
The largest term is 22.
-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1+0+1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22 = 100
Edwin
Question 560377: what is the next number in the arithmetic sequence 3, 11, 19, 29 Answer by kkasko(45) (Show Source):
You can put this solution on YOUR website!what is the next number in the arithmetic sequence 3, 11, 19, 29...
Are you sure the 29 is not supposed to be 27?
3+8=11+8=19+8=27
To solve you should first find the common difference.
d=t4-t3
d=27-19
d=8
plug it into the formula tn=t1+(n-1)d
t5=3+(5-1)8
t5=3+4(8)
t5=3+32
t5=32
3,11,19,27,32...
Question 559368: what's next in this sequence? 1, 6, 12, 17, 34, 39, 78... Answer by richard1234(4796) (Show Source):
Question 559630: what would the 7th pattern be to the following sequence: 1, 1+4=5, 1+4+9=14 Answer by solver91311(12126) (Show Source):
Question 559371: what's next 5 numbers in this sequence? 1, 6, 12, 17, 34, 39, 78... Answer by KMST(592) (Show Source):
You can put this solution on YOUR website!83, 166, 171, 342, 347
Adding 11 to the 1st and 2nd numbers you get the 3rd and 4th respectively.
1+11=12, 6+11=17
Adding 22 to the 3rd and 4th numbers you get the 5th and 6th respectively.
12+22=34, 17+22=39
Adding 44 to the 5th and 6th numbers you get the 7th and 8th respectively.
34+44=78, 39+44=83
Adding 88 to the 7th and 8th numbers you get the 9th and 10th respectively.
78+88=166, 83+88=171
Adding 176 to the 9th and 10th numbers you get the 11th and 12th respectively.
166+176=342, 171+176=347
Question 559241: cube root of 44.5 Answer by rfer(10417) (Show Source):
Question 558951: Try to solve this sequence
112 , 2112 , 2122 ....
Calculate the further terms Answer by Edwin McCravy(6936) (Show Source):
You can put this solution on YOUR website!
t1 = 112
t2 = 2112
t3 = 2122
Three terms always determine a quadratic sequence.
So let the nth term be
tn = an² + bn + c
Then
t1 = 112 = a(1)² + b(1) + c
a + b + c = 112
t2 = 2112 = a(2)² + b(2) + c
4a + 2b + c = 2112
t3 = 2122 = a(3)² + b(3) + c
9a + 3b + c = 2122
So we solve the system:
ì a + b + c = 112
í4a + 2b + c = 2112
î9a + 3b + c = 2122
We get a=-995, b = 4985, and c = -3878
So the three terms given fit the sequence:
tn = -995n² + 4985t - 3878
So the first 50 terms are:
t1 = -995(1)² + 4985(1) - 3878 = 112
t2 = -995(2)² + 4985(2) - 3878 = 2112
t3 = -995(3)² + 4985(3) - 3878 = 2122
t4 = -995(4)² + 4985(4) - 3878 = 142
t5 = -995(5)² + 4985(5) - 3878 = -3828
t6 = -995(6)² + 4985(6) - 3878 = -9788
t7 = -995(7)² + 4985(7) - 3878 = -17738
t8 = -995(8)² + 4985(8) - 3878 = -27678
t9 = -995(9)² + 4985(9) - 3878 = -39608
t10 = -995(10)² + 4985(10) - 3878 = -53528
t11 = -995(11)² + 4985(11) - 3878 = -69438
t12 = -995(12)² + 4985(12) - 3878 = -87338
t13 = -995(13)² + 4985(13) - 3878 = -107228
t14 = -995(14)² + 4985(14) - 3878 = -129108
t15 = -995(15)² + 4985(15) - 3878 = -152978
t16 = -995(16)² + 4985(16) - 3878 = -178838
t17 = -995(17)² + 4985(17) - 3878 = -206688
t18 = -995(18)² + 4985(18) - 3878 = -236528
t19 = -995(19)² + 4985(19) - 3878 = -268358
t20 = -995(20)² + 4985(20) - 3878 = -302178
t21 = -995(21)² + 4985(21) - 3878 = -337988
t22 = -995(22)² + 4985(22) - 3878 = -375788
t23 = -995(23)² + 4985(23) - 3878 = -415578
t24 = -995(24)² + 4985(24) - 3878 = -457358
t25 = -995(25)² + 4985(25) - 3878 = -501128
t26 = -995(26)² + 4985(26) - 3878 = -546888
t27 = -995(27)² + 4985(27) - 3878 = -594638
t28 = -995(28)² + 4985(28) - 3878 = -644378
t29 = -995(29)² + 4985(29) - 3878 = -696108
t30 = -995(30)² + 4985(30) - 3878 = -749828
t31 = -995(31)² + 4985(31) - 3878 = -805538
t32 = -995(32)² + 4985(32) - 3878 = -863238
t33 = -995(33)² + 4985(33) - 3878 = -922928
t34 = -995(34)² + 4985(34) - 3878 = -984608
t35 = -995(35)² + 4985(35) - 3878 = -1048278
t36 = -995(36)² + 4985(36) - 3878 = -1113938
t37 = -995(37)² + 4985(37) - 3878 = -1181588
t38 = -995(38)² + 4985(38) - 3878 = -1251228
t39 = -995(39)² + 4985(39) - 3878 = -1322858
t40 = -995(40)² + 4985(40) - 3878 = -1396478
t41 = -995(41)² + 4985(41) - 3878 = -1472088
t42 = -995(42)² + 4985(42) - 3878 = -1549688
t43 = -995(43)² + 4985(43) - 3878 = -1629278
t44 = -995(44)² + 4985(44) - 3878 = -1710858
t45 = -995(45)² + 4985(45) - 3878 = -1794428
t46 = -995(46)² + 4985(46) - 3878 = -1879988
t47 = -995(47)² + 4985(47) - 3878 = -1967538
t48 = -995(48)² + 4985(48) - 3878 = -2057078
t49 = -995(49)² + 4985(49) - 3878 = -2148608
t50 = -995(50)² + 4985(50) - 3878 = -2242128
Edwin
Question 558856: 1/4,1/4,3/4,5/4
I tried to find the pattern but the two 1/4s got me please help Answer by stanbon(48545) (Show Source):
Question 558751: What is the seventh term in this sequence
6,-18,54,-163...
Thank you for your help I am really stuck. Answer by scott8148(5880) (Show Source):
You can put this solution on YOUR website!6 times -3 is -18
-18 times -3 is 54
54 times -3 is -162
it appears that there is an error in the recording of the fourth term
___ also, the first three terms are even but the fourth term is odd
you can follow the pattern to find the seventh term
Question 558677: if the pattern 1 3 7 is contiued what would be the 5th term (1 3 7 ? ?) Answer by unlockmath(1122) (Show Source):
Question 558345: The sum of the squares of two consecutive even integers is 1684. Find the two integers. Answer by Alan3354(21583) (Show Source):
You can put this solution on YOUR website!The sum of the squares of two consecutive even integers is 1684. Find the two integers.
-----------
 = apx 29, the center
--> 28 & 30 (plus and minus)
---------------
The hard way:

--> same answer
Question 558346: The lengths of the three sides of a right triangle are given by three consecutive even integers. Find the lengths of the three sides. Answer by Alan3354(21583) (Show Source):
You can put this solution on YOUR website!The lengths of the three sides of a right triangle are given by three consecutive even integers. Find the lengths of the three sides.
-----------
It's 6, 8 & 10, 2x the 3,4,5 right triangle.
---------

You can solve that and get the answer.
 is easier, tho.
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Older solutions: 1..45, 46..90, 91..135, 136..180, 181..225, 226..270, 271..315, 316..360, 361..405, 406..450, 451..495, 496..540, 541..585, 586..630, 631..675, 676..720, 721..765, 766..810, 811..855, 856..900, 901..945, 946..990, 991..1035, 1036..1080, 1081..1125, 1126..1170, 1171..1215, 1216..1260, 1261..1305, 1306..1350, 1351..1395, 1396..1440, 1441..1485, 1486..1530, 1531..1575, 1576..1620, 1621..1665, 1666..1710, 1711..1755, 1756..1800, 1801..1845, 1846..1890, 1891..1935, 1936..1980, 1981..2025, 2026..2070, 2071..2115, 2116..2160, 2161..2205, 2206..2250, 2251..2295, 2296..2340, 2341..2385, 2386..2430, 2431..2475, 2476..2520, 2521..2565, 2566..2610, 2611..2655, 2656..2700, 2701..2745
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