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robertb answered: 4011 problems
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Probability-and-statistics/401617: If you have a six-sided dice and you roll that dice 90 times, what is the expected probability your dice will land on a 2 and a 3?
1 solutions

Answer 284214 by robertb(4012) About Me  on 2011-01-29 23:35:56 (Show Source):
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1)die = singular, dice = plural
2) The probability that a die, thrown once, will show 2 AND 3 is 0.
The probability that a die, thrown once, will show 2 OR 3 is 2/6, or 1/3.
Thus the expected number of times a 2 or a 3 will show is (1/3)*90 =30 times.


Polynomials-and-rational-expressions/401615: solve and write in interval notation: -3/2 < 4-2m < 7/2
1 solutions

Answer 284207 by robertb(4012) About Me  on 2011-01-29 23:09:56 (Show Source):
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-3/2 - 4 < -2m < 7/2 - 4
==> -11/2 < -2m < -1/2
==> 11/4 > m > 1/4, or (1/4, 11/4) in interval notation.


Graphs/401356: My 14 year old son has Asperger Syndrome and, up until this year, has always excelled in math, scoring very high in California Achievement Tests and working above grade level. I home school my children and have used various math programs, looking for the best fit (I have several on the autism spectrum and another with ADHD). My son is having a horrible time with Algebra 1 (which many children on the spectrum do) and I am hoping for any advice you can give. We have tried Kinetic textbooks, but it was not a good fit. We are now using Math U See, which seems to work, but he is still struggling...we are working on slope/standard formula and graphing at the moment. I would appreciate any advice you could give on teaching Algebra to someone who is a very concrete thinker so that I can get him through this. Thank you!
1 solutions

Answer 284062 by robertb(4012) About Me  on 2011-01-29 07:24:24 (Show Source):
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Sir/Madam,
The basis of all graphing exercises, from the simplest (plotting points) to the the most complicated (e.g., algebraic geometry), is an appreciation of the basic Cartesian coordinate system. Give your son lots of exercises on plotting random points on the Cartesian system. Then ask him if he can form geometrical objects, any geometrical object, by connecting dots (points) on the coordinate system. After doing this for some time (probably after having had enough of this simple exercise), introduce order and structure by illustrating how to form a straight line using two points (x%5B1%5D, y%5B1%5D) and (x%5B0%5D, y%5B0%5D). Always keep in mind the position of the line with respect to the origin (0,0), and whether the line falls or rises from left to right. Don't be discouraged if he doesn't get it right away: for many people it takes time to understand the most basic of mathematical concepts (which might actually include me.) =)


Geometry_proofs/401390: Prove that 2TanA/1+Tan^2A=Sin2A
1 solutions

Answer 284058 by robertb(4012) About Me  on 2011-01-29 06:45:31 (Show Source):


Inequalities/401391: Hi! Could you solve this inequalitiy and show me the steps please?
2%2F%28-x%2B3%29%3C=2%2F%28-x%29
Thank you!
1 solutions

Answer 284057 by robertb(4012) About Me  on 2011-01-29 06:36:11 (Show Source):
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2%2Fx+-+2%2F%28x-3%29%3C=+0 <==> 1%2Fx+-+1%2F%28x-3%29%3C=+0
<==> %28x+-+3+-+x%29%2F%28x%28x-3%29%29+%3C=+0 ==> 1%2F%28x%28x-3%29%29+%3E=+0
The critical numbers of the inequality are 0 and 3.
At the interval (-infinity, 0), 1%2F%28x%28x-3%29%29+%3E=+0.
At the interval (0,3), 1%2F%28x%28x-3%29%29+%3C+0.
At the interval (3, infinity), 1%2F%28x%28x-3%29%29+%3E=+0.
The critical numbers 0 and 3 don't satisfy the inequality.
Therefore the solution set is (-infinity, 0)U(3, infinity).


Probability-and-statistics/401386: Twelve dice are cast. What is the probability that each of the six faces will appear at least once?
1 solutions

Answer 284056 by robertb(4012) About Me  on 2011-01-29 05:50:23 (Show Source):


Trigonometry-basics/400656: an equilateral triangle has side lengths of 8. what is the lenght of its altitude
1 solutions

Answer 284043 by robertb(4012) About Me  on 2011-01-29 03:26:49 (Show Source):


Probability-and-statistics/400727: There are 9 marbles in a bag. Some are red, some are white, and some are blue. The probability of selecting a red marble, a white marble, and then a blue marble is 1/21. How many of each color are in the bag?
1 solutions

Answer 284042 by robertb(4012) About Me  on 2011-01-29 03:21:36 (Show Source):
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Without replacement:
%28r%2F9%29+%2A+%28w%2F8%29+%2A+%28b%2F7%29+=+1%2F21 ==> rwb = 24 = 2*3*4.
Since r+w+b = 9, there 6 possibilities for the ordered triple(r,w,b):
(2,3,4)
(2,4,3)
(3,2,4)
(3,4,2)
(4,3,2)
(4,2,3)

With replacement:
%28r%2F9%29+%2A+%28w%2F9%29+%2A+%28b%2F9%29+=+1%2F21==> rwb+=+243%2F7, which is impossible, because r,w, b are positive integers satisfying r+ w + b = 9


Probability-and-statistics/400649: A committee of 12 students must select from their group a president, a vice-president, and a secretary. In how many possible ways can this be accomplished?
1 solutions

Answer 284041 by robertb(4012) About Me  on 2011-01-29 03:07:28 (Show Source):
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12*11*10 = 1,320 ways


Linear-systems/400641: line 1: y= -1/4(x) -1
line 2: x+4y=-4
Is this system of the equation
a) consistent independent
b) consistnt dependent
c) inconsistent
This means the system has
a) a unique solution (_ , _)
b) infinitly many solutions
c) no solutions'
1 solutions

Answer 284040 by robertb(4012) About Me  on 2011-01-29 03:04:31 (Show Source):
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Consistent dependent, meaning infinitely many solutions.


Linear_Algebra/400630: let{u1,u2....uk}be a linearly independent set of vector in R^n and c1,c2,....ck are scalars. prove that {c1u1, c2u2....., ckuk}is also limearly independent.
1 solutions

Answer 284039 by robertb(4012) About Me  on 2011-01-29 02:58:21 (Show Source):
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Since { u%5B1%5D,+u%5B2%5D, u%5B3%5D, ..., u%5Bk%5D } are linearly independent, then the equation d%5B1%5Du%5B1%5D+%2B+d%5B2%5Du%5B2%5D+%2B+d%5B3%5Du%5B3%5D+ ...+ d%5Bk%5Du%5Bk%5D+=+theta has only the trivial solution d%5B1%5D=+d%5B2%5D+=+d%5B3%5D= ...= d%5Bk%5D+=+0. (theta+is the zero vector, and d%5Bn%5D's are scalars.)
We have to assume that none of the c%5Bn%5D's are equal to zero, otherwise,
{ c%5B1%5Du%5B1%5D,+c%5B2%5Du%5B2%5D, c%5B3%5Du%5B3%5D, ..., c%5Bk%5Du%5Bk%5D } would contain the zero vector, automatically making the set linearly dependent. Hence
+ ...+ %28d%5Bk%5D%2Fc%5Bk%5D%29%2A%28c%5Bk%5Du%5Bk%5D%29+=+theta
==> d%5Bn%5D%2Fc%5Bn%5D+=+0 for every n = 1,2,3,...,k, and { c%5B1%5Du%5B1%5D,+c%5B2%5Du%5B2%5D, c%5B3%5Du%5B3%5D, ..., c%5Bk%5Du%5Bk%5D } is a linearly independent set.



Trigonometry-basics/400604: u=2i-5j,v=-3i+7j and w=-i-6j
find each specified vector or scalar
4w-3v
1 solutions

Answer 284036 by robertb(4012) About Me  on 2011-01-29 02:30:40 (Show Source):
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4w - 3v = 4(-i-6j) - 3(-3i + 7j) = -4i - 24j + 9i - 21j = 5i - 45j


Travel_Word_Problems/401299: Find the length of the diagonal and the area of a square with perimeter equal to 1.2√2 inches.
1 solutions

Answer 284022 by robertb(4012) About Me  on 2011-01-28 23:40:04 (Show Source):
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Area = %28p%2F4%29%5E2+=+%281.2sqrt%282%29%2F4%29%5E2+=+%280.3sqrt%282%29%29%5E2+=+0.18 square inch.
Diagonal = %28p%2F4%29%2Asqrt%282%29+=+%280.3sqrt%282%29%29%2Asqrt%282%29+=+0.6 inch.


Probability-and-statistics/401312: What is the critical value for the following Chi- square test? A manufacturer has 4 locations and is looking to see if shipments with defects are dependent on the manufacturer location. The defects are categorized in two rows (with defects and no defects). The researcher is testing using a 95% confidence level. Write the null and alternative hypothesis for the scenario.
1 solutions

Answer 284020 by robertb(4012) About Me  on 2011-01-28 23:32:40 (Show Source):
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H%5Bo%5D: Defects are independent of manufacturer location
H%5Ba%5D: Defects are associated with manufacturer location
p = 0.05, with degrees of freedom = df = (2 - 1)(4 - 1) = 3.
Critical value is 7.82.


Inequalities/401358: How would I solve and graph the compound inequality. Thank you
7x+4≥-24 or 7x+1≥-13
1 solutions

Answer 284017 by robertb(4012) About Me  on 2011-01-28 23:24:52 (Show Source):
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7x+%3E=+-28 <==> x+%3E=+-4
7x+%3E+=+-14 <==> x+%3E=+-2
==> {x:x+%3E=+-4 }U {x:x+%3E=+-2} = {x:x+%3E=+-4}


Miscellaneous_Word_Problems/401346: Pablo's gas tank is 1/2 full. After he buys 7 gallons of gas, it is 5/6 full. How many gallons can Pablo's tank hold?

1 solutions

Answer 284014 by robertb(4012) About Me  on 2011-01-28 23:05:29 (Show Source):
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x%2F2+%2B+7+=+5x%2F6 ==> 7+=+x%2F3 ==> x = 21, the total number of gallons Pablo's tank can hold.


Miscellaneous_Word_Problems/401294: Rob can complete his bus route in 4 hours. Joan can complete her route in 2 hours. If they both leave the terminal at 5:00 am and after each completed route return to the terminal, when is the next time they both leave the terminal at the same time?
1 solutions

Answer 284013 by robertb(4012) About Me  on 2011-01-28 22:57:20 (Show Source):


Equations/401144: Angle A is acute and such that tanA=√6/3.
(a) Show clearly that the exact value of sin A can be written in the form 1/5√k and state the value of k.
(b) Hence, or otherwise, show that the value of cos2A is exactly 1/5
1 solutions

Answer 284011 by robertb(4012) About Me  on 2011-01-28 22:45:32 (Show Source):
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(a)
==> .
(b)cos+%282A%29+=+1-2sin%5E2%28A%29+=+1+-+2%2A%2810%2F25%29+=+1+-+4%2F5+=+1%2F5


Polynomials-and-rational-expressions/401340: factor completly u^3-125v^3
1 solutions

Answer 284008 by robertb(4012) About Me  on 2011-01-28 22:34:10 (Show Source):


Inequalities/401339: 5[3m-(m+6)]>-6(m-5)
I need to use set-builder notations to describe the complete solution.{m/m__ __}
Thanks for the help!
1 solutions

Answer 284003 by robertb(4012) About Me  on 2011-01-28 22:31:22 (Show Source):
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15m - 5(m+6) > -6m + 30,
15m - 5m - 30 > -6m + 30,
10m - 30 > -6m + 30,
16m > 60,
m > 60/16 = 15/4,
or {m| m > 15/4}.


Equations/401136: Express the function f(x)=5cos(2x)°-3sin(2x)° in the form f(x)=kcos(2x-α)°,
where k>0 and 0≤α≤360
1 solutions

Answer 284002 by robertb(4012) About Me  on 2011-01-28 22:27:20 (Show Source):
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Let cos%28alpha%29+=+5%2Fsqrt%2834%29 and sin%28alpha%29+=+-3%2Fsqrt%2834%29, where
alpha+ is in the 4th quadrant.
Then
5cos+%282x%29+-+3sin%282x%29+=+sqrt%2834%29%2Acos%282x+-+alpha%29


Radicals/401298: Find (if it exists) each function value of f(t)=the square root of t^2+1 and so f(-6)=?
1 solutions

Answer 284001 by robertb(4012) About Me  on 2011-01-28 22:19:46 (Show Source):
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If the function is f%28t%29+=+sqrt%28t%5E2%2B1%29, then f(-6) exists, and is equal to f%28-6%29+=+sqrt%28%28-6%29%5E2%2B1%29+=+sqrt%2837%29.


Equations/401332: solve the equation 1/(x+3) - 1/(x+4) = 1/2
1 solutions

Answer 283999 by robertb(4012) About Me  on 2011-01-28 22:12:40 (Show Source):
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+1%2F%28x%2B3%29+-+1%2F%28x%2B4%29+=+1%2F2
==> 2%28%28x%2B4%29-%28x%2B3%29%29=%28x%2B3%29%28x%2B4%29
==> 2+=+x%5E2+%2B+7x+%2B+12
==> 0+=+x%5E2+%2B+7x+%2B+10
<==> 0 = (x + 2)(x+ 5) ==> x = -2, -5.
Both values will never make the denominator 0, so they are the final answers.


Quadratic_Equations/401141: During the first part of a trip a canoeist travels 48 miles at a certain speed. The canoeist travels 19 miles on the second part of the trip at a speed 5 mph slower. The total time for the trip is 3 hours. What was the speed on each part of the trip?
1 solutions

Answer 283995 by robertb(4012) About Me  on 2011-01-28 22:00:53 (Show Source):
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Let r = speed at the 1st part of trip ==> time for 1st part = 48/r
==> r - 5 = speed at 2nd part of trip ==> time for 2nd part = 19/(r-5).
both from the formula t = d/r = distance/rate.
Then 48%2Fr+%2B+19%2F%28r-5%29+=+3
<==> 48(r-5) + 19r = 3r(r-5)
<==> 48r+-+240+%2B+19r+=+3r%5E2+-+15r
<==> 3r%5E2+-+82r+%2B+240+=+0 <==> (r - 24)(3r - 10) = 0
==> r = 24, 10/3
Eliminate r = 10/3, because it would make r - 5 negative.
Hence the speed in the 1st part is 24 mph, and the speed in the 2nd part is 19 mph.


Functions/400822: what is the domain of √ ̅ (x-3) divided by x^2-8x+12?
1 solutions

Answer 283732 by robertb(4012) About Me  on 2011-01-27 21:02:41 (Show Source):
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If your rational function is sqrt%28%28x-3%29%2F%28x%5E2-8x%2B12%29%29+=+sqrt%28%28x-3%29%2F%28%28x-2%29%28x-6%29%29%29, then the domain is obtained as follows:
From the expression %28x-3%29%2F%28x%5E2-8x%2B12%29+=+%28x-3%29%2F%28%28x-2%29%28x-6%29%29, the critical numbers are 2, 3, and 6. These critical numbers partition the real number line into the intervals (-infinity, 2), (2, 3), (3, 6), (6, infinity).
At (-infinity, 2), %28x-3%29%2F%28%28x-2%29%28x-6%29%29+%3C+0 by using the test point x = 0.
At (2, 3), %28x-3%29%2F%28%28x-2%29%28x-6%29%29+%3E+0 by using the test point x = 2.5.
At (3, 6), %28x-3%29%2F%28%28x-2%29%28x-6%29%29+%3C+0 by using the test point x = 4.
At (6, infinity), %28x-3%29%2F%28%28x-2%29%28x-6%29%29+%3E+0 by using the test point x = 7.
The critical numbers 2 and 6 are not included in the domain, as these will make the denominator equal to 0. We want those intervals that will make %28x-3%29%2F%28%28x-2%29%28x-6%29%29+%3E+0, because the whole expression is under the square root symbol. The critical number x = 3 is included in the domain.
Hence the domain of the rational function is (2,3]U(6, infinity).


Trigonometry-basics/400480: Given the following values, the value of c is ________.
b = 4.5 ft
a = 10 ft
C = 105
1 solutions

Answer 283497 by robertb(4012) About Me  on 2011-01-27 00:43:39 (Show Source):
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Cosine Law: c%5E2+=+a%5E2+%2B+b%5E2+-+2abcosC
==> c%5E2+=+10%5E2+%2B+4.5%5E2+-+2%2A10%2A4.5cos105
==> c%5E2+=+143.5437141 ==> c = 11.981, to 3 decimal places.


Graphs/400494: What is a slope of a line?
1 solutions

Answer 283488 by robertb(4012) About Me  on 2011-01-27 00:21:37 (Show Source):
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Measure of steepness of a line = Rise/Run = %28y%5B1%5D+-+y%5B0%5D%29%2F%28x%5B1%5D+-x%5B0%5D%29 for two points (x%5B0%5D,+y%5B0%5D) and (x%5B1%5D,+y%5B1%5D) on the line.


Linear-equations/400510: Find the equation of the line passing through (-3, 4) and parallel to 2x + 5y = 12.
1 solutions

Answer 283487 by robertb(4012) About Me  on 2011-01-27 00:17:13 (Show Source):
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The line must have the equation 2x + 5y = c. To find c, use the coordinates of the point (-3,4):
2*-3 + 5*4 = -6 + 20 = 14
Therefore the equation of the line is 2x + 5y = 14.


Trigonometry-basics/400477: Given the following values, how many triangles are there?
a = 16
b = 18
A = 60




















1 solutions

Answer 283477 by robertb(4012) About Me  on 2011-01-26 22:40:47 (Show Source):
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2 triangles are possible.
From the sine law, sinA%2Fa+=+sinB%2Fb, we getsinB+=+%2818sin60%5Eo%29%2F16+=+0.97248 to 5 decimal places.
==> B+=+76%5Eo58'36"
1st possibility:
A = 60%5Eo
B = +76%5Eo58'36"
C = 43%5Eo1'24"
2nd possibility:
A = 60%5Eo
B = 180%5Eo - +76%5Eo58'36" = 103%5Eo1'24"
C = 16%5Eo58'36"
Just get the sides using several applications of the sine law.


Miscellaneous_Word_Problems/400475: speed is 20mph leave at 10:20am how for will you have gone by 5:00 pm
1 solutions

Answer 283473 by robertb(4012) About Me  on 2011-01-26 22:27:09 (Show Source):


Percentage-and-ratio-word-problems/400342: The percent of pure gold in 14-karat gold is about 58.3%. A 14-
karat gold ring has a mass of 5.6 grams. About how many grams of pure
gold are in the ring?
1 solutions

Answer 283470 by robertb(4012) About Me  on 2011-01-26 22:23:54 (Show Source):
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5.6*0.583 =3.2648 grams of pure gold.