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richard1234 answered: 5385 problems
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Geometry_proofs/431819: Prove nCr = n/(n-r) * n-1Cr using an analytic and combinatoial proof
1 solutions

Answer 299569 by richard1234(5390) About Me  on 2011-04-05 23:06:03 (Show Source):
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The right hand side is equal to

%28n%2F%28n-r%29%29%28%28n-1%21%29%2F%28n-1-r%29%21r%21%29

Since n%28n-1%29%21+=+n%21, and %28n-r%29%28n-1-r%29%21+=+%28n-r%29%21 the right hand side is equal to

n%21%2F%28n-r%29%21r%21, same as the left hand side.


Equations/431771: Can you please explain how you would work this problem out I just don't understand the whol please excuse my dear aunt sally, but since multiplication goes before division why is it that on this equation you will have to divide before you do your multiplication. Im so confused....please help me understand this....
35(41-33)÷8-11+17

1 solutions

Answer 299568 by richard1234(5390) About Me  on 2011-04-05 23:03:28 (Show Source):
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Actually you have to multiply first before dividing:

35(41 - 33) ÷ 8 - 11 + 17
35(8) ÷ 8 - 11 + 17
35 - 11 + 17
= 41

Multiplying and dividing typically take equal precedence over each other, and in such a case you would simply go from left to right

(I checked WolframAlpha for its answer, it seemed as if it divided first, but both WolframAlpha and I got 41. This is mainly because the order of these operations usually doesn't matter, i.e. a%28b%2Fc%29+=+%28ab%29%2Fc).


Complex_Numbers/431787: If i=the square root of _1, what is then 4i(6i)=

SHOW ME THE STEPS SO THAT I CAN DO THE OTHERS BY MY SELF
1 solutions

Answer 299566 by richard1234(5390) About Me  on 2011-04-05 22:57:07 (Show Source):


Square-cubic-other-roots/431789: I can't find this answer anywhere please help
what is the inverse of raising a quantity to a power?
1 solutions

Answer 299560 by richard1234(5390) About Me  on 2011-04-05 22:41:48 (Show Source):
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The inverse of taking a quantity to the nth power is taking the nth root of that quantity.


Geometry_proofs/431252: I am having trouble with this proof
If given a line segment like ab
Construct a perpendicular line (i can do)
Prove that the line is perpendicular (i can't figure out)
I need a proof for a perpendicular bisector of a line.

Thank you!!
Mary
1 solutions

Answer 299535 by richard1234(5390) About Me  on 2011-04-05 21:01:10 (Show Source):
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Two perpendicular lines simply mean that one line is perpendicular to the other.


Complex_Numbers/431744: Can you please tell me the answer to 5i to the fourth power? I think it is 625. My thought is, if "i" is involved, and a number is to an even power, then it is a positive number. If it is to an odd power, then it is a negative number.
1 solutions

Answer 299533 by richard1234(5390) About Me  on 2011-04-05 20:58:18 (Show Source):
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It is 625. But i to an even power is not always positive, and i to an odd number is always imaginary. Basically,

i%5E0+=+1
i%5E1+=+i
i%5E2+=+-1
i%5E3+=+i%28i%5E2%29+=+-i
i%5E4+=+1
i%5E5+=+i and so on.


sets-and-operations/431739: Given these sets:
A = {3, 4, 5, 6, 7}
B = {6, 7, 8, 9}
C = {2, 4, 6, 8}

List the elements of this set
C ∩ (AUB)
Do not skip to the answer. Show your work
1 solutions

Answer 299522 by richard1234(5390) About Me  on 2011-04-05 20:43:22 (Show Source):
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A U B = {3, 4, 5, 6, 7, 8, 9} (simply take all elements in either A or B)

C ∩ (A U B) = {4, 6, 8} (these are the elements in both C and A U B)


Complex_Numbers/431413: Why does ei*pi+1=0 other than the fact that my calculator says so, and so I believe it?
1 solutions

Answer 299519 by richard1234(5390) About Me  on 2011-04-05 20:41:07 (Show Source):
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Apparently it seems like the other tutor taught you this...:)

From Euler's formula, e%5E%28i%2Atheta%29+=+cos%28theta%29+%2B+i%2Asin%28theta%29. By substituting theta+=+pi you will obtain e%5E%28i%2Api%29+=+-1, e%5E%28i%2Api%29+%2B+1+=+0. Euler's formula itself is derived using the Maclaurin series for e%5Ex, namely e%5Ex+=+sum%28%28x%5Ek%29%2Fk%21%2C+k=0%2C+infinity%29 (you will learn this sometime in calculus).


Trigonometry-basics/431693: how do you solve this help show me please so i can understand
1 solutions

Answer 299513 by richard1234(5390) About Me  on 2011-04-05 20:35:28 (Show Source):
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"how do you solve this help show me please so i can understand"

What is "this?"


absolute-value/431711: how do you find the absolute value when it is on a number line ?
1 solutions

Answer 299512 by richard1234(5390) About Me  on 2011-04-05 20:33:58 (Show Source):
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The absolute value is the positive distance between that number and zero.


Word_Problems_With_Coins/152564: how many ways to make change for one dollar using nickels, dimes and quarters
1 solutions

Answer 299461 by richard1234(5390) About Me  on 2011-04-05 16:12:32 (Show Source):
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You can compute the number of ways to make change for $1 using a bijection. We let n, d, q be the number of nickels, dimes, and quarters, and 5n+%2B+10d+%2B+25q+=+100, which implies n+%2B+2d+%2B+5q+=+20. Hence, this is equivalent to finding the number of ways to make 20 "cents" using one-, two-, and five-cent pieces.

Suppose we want to find the number of ways to make 20 cents *without* using a five cent piece.

Case 0: We wish to make 0 cents --> 1 way (just use no coins)
Case 1: We wish to make 1 cent --> 1 way 2*0 + 1*1
Case 2: We wish to make 2 cents --> 2 ways 2*1 + 1*0; 2*0 + 1*2
Case 3: We wish to make 3 cents --> 2 ways 2*1 + 1*1; 2*0 + 1*3
Case 4: We wish to make 4 cents --> 3 ways 2*2 + 1*0; 2*1 + 1*2, 2*0 + 1*4
Case 5: We wish to make 5 cents --> 3 ways 2*2 + 1*1, 2*1 + 1*3, 2*0 + 1*5

We can prove using either induction or modular arithmetic that the number of ways to make n cents in this way is floor%28n%2F2%29+%2B+1, where floor%28x%29 denotes the floor value of x. We can evaluate this at n = 20 to get floor%2820%2F2%29+%2B+1, or 11.

Similarly, we can count the number of ways to obtain 20 cents using one five-cent piece. However, we can subtract off the five-cent piece and say that this is analogous to computing the number of ways to obtain 15 cents. Hence, this is equal to floor%2815%2F2%29+%2B+1+=+8.

For 10, 5, and 0 cents, we have floor%2810%2F2%29+%2B+1+=+6, floor%285%2F2%29+%2B+1+=+3 and floor%280%2F2%29+%2B+1+=+1. Therefore the total number of ways is 11 + 8 + 6 + 3 + 1 = 29 ways.


Note: the other tutor counted 293 ways, however this included pennies and half dollars, which was not stated in the question. To count the number of ways using pennies, nickels, dimes, and quarters, you can also use a bijection, but instead of counting the number of ways to obtain 20 cents, you evaluate at 20, 19, 18, ..., 0 since this will uniquely determine the number of pennies. We do the same with 15, 10, 5, and 0 to obtain sums:



Then, if you want half dollars involved, it is equal to 242+%2B+k%5B1%5D+%2B+k%5B2%5D where k%5Bi%5D is the number of ways to make a dollar using i half dollars, equivalently, the number of ways to make 50 cents and 0 cents without half dollars (here you should get 293!).


Volume/431466: what does it mean to say that an object has a volume of 6 cubic inches?
1 solutions

Answer 299460 by richard1234(5390) About Me  on 2011-04-05 15:45:39 (Show Source):
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It can hold six cubic inches of something, say water. One cubic inch is the volume of water in a 1" x 1" x 1" container.


Trigonometry-basics/431469: Derive the half angle formula for sin( u/2 ) by following the steps below.
(a) Use the sum angle formula to write cos(2x) in terms of powers of sin(x)
and cos(x) .
(b) Use a trig identity to rewrite cos(2x) in terms of just powers of sin(x):
(c) Solve the formula in part (b) in terms of sin(x), and not a power of
sin(x).
(d) Substitute x = u/2 into the formula in part (c).
1 solutions

Answer 299459 by richard1234(5390) About Me  on 2011-04-05 15:43:05 (Show Source):
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cos%282x%29+-+1+=+-2sin%5E2%28x%29

sin%5E2%28x%29+=+%281+-+cos%282x%29%29%2F2

sin%28x%29+=+sqrt%28%281+-+cos%282x%29%29%2F2%29

Replacing x with u/2,

sin%28u%2F2%29+=+sqrt%28%281+-+cos%28u%29%29%2F2%29 (however you must be careful what quadrant u and u/2 are in, because it could be positive or negative).


Linear-equations/431524: Why should we clear fractions when solving linear equations and inequalities? Can you give me an example?
Why should we clear decimals when solving linear equations and inequalities? Can you give me an example.
1 solutions

Answer 299456 by richard1234(5390) About Me  on 2011-04-05 15:36:48 (Show Source):
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If you're given an equation or inequality such as x%2F3+%2B+5+=+2, it helps to clear the fraction or decimal since we are trying to find the value of x, not x%2F3.


Inequalities/431528: Why does the inequality sign change when both sides are multiplied or divided by a negative number?
1 solutions

Answer 299455 by richard1234(5390) About Me  on 2011-04-05 15:34:38 (Show Source):
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Suppose a+%3E+b, and n+%3E+0. From a+%3E+b we obtain a+-+b+%3E+0. We can subtract a-b from both sides to get 0+%3E+-%28a-b%29 --> 0+%3E+-a+%2B+b --> -b+%3E+-a. Finally, we can scale both sides by the same positive number n to get -bn+%3E+-an.


Expressions-with-variables/431476: Use the commutative law of addition to write an equivalent expression.
f + 6g

1 solutions

Answer 299454 by richard1234(5390) About Me  on 2011-04-05 15:30:35 (Show Source):


test/431296: whar is the probability that a ramdomly selected day in january is after the 20th?
1 solutions

Answer 299376 by richard1234(5390) About Me  on 2011-04-05 00:11:18 (Show Source):
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Out of the days {21, 22, ..., 31} there are 11 days (inclusive). Therefore the probability is 11/31.


Equations/431282: Find the quadratic equation with integral coefficients having the given roots.
4 + 2i, 4 - 2i
Thanks!!
1 solutions

Answer 299375 by richard1234(5390) About Me  on 2011-04-05 00:10:20 (Show Source):
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The quadratic will have x+-+%284%2B2i%29+=+x+-+4+-+2i and x+-+%284-2i%29+=+x+-+4+%2B+2i as factors. Hence, the quadratic is equal to (applying the difference of two squares)

Expanding and simplifying, this is equal to x%5E2+-+8x+%2B+16+%2B+4+=+x%5E2+-+8x+%2B+20.


Rectangles/431308: how do you find the area of a rectangular prism? do i multiply the sides or add?
1 solutions

Answer 299374 by richard1234(5390) About Me  on 2011-04-04 23:43:32 (Show Source):


Radicals/431151: I am trying to simplify radical expressions.. It says Assume that the variables are positive. Write "nonreal" if the expression does not represent a real number. The problem is sqrt( 98 ). I know the answer is 7 sqrt ( 2 ) but I am not sure how I got that answer. How do I get from sqrt ( 98 ) to 7 sqrt (2) ?
Thanks!
1 solutions

Answer 299306 by richard1234(5390) About Me  on 2011-04-04 19:17:57 (Show Source):
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Factor out the largest square divisor of 98. Since 98+=+7%5E2%2A2, sqrt%2898%29+=+sqrt%287%5E2%2A2%29+=+sqrt%287%5E2%29sqrt%282%29+=+7sqrt%282%29.


logarithm/431013: what is the derivative of:
1. y= tanx + 3
2. y= 7x - 5 sinx - secx ????
1 solutions

Answer 299304 by richard1234(5390) About Me  on 2011-04-04 19:00:53 (Show Source):
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Simply apply the basic rules of differentiation, as well as the derivatives of various trigonometric functions:

1. dy%2Fdx+=+sec%5E2+%28x%29

2. dy%2Fdx+=+7+-+5cos%28x%29+-+sec%28x%29tan%28x%29


testmodule/430948: how many moles of HCl we get for mixing 1 mole of hydrogen and 1000000 moles of chlorine??
1 solutions

Answer 299303 by richard1234(5390) About Me  on 2011-04-04 18:54:24 (Show Source):
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I would suppose you would get 1 mole, since the other 999,999 moles of chlorine would not react.


Quadratic_Equations/431049: write a quadratic equation that has the solutions of -2 and 5
1 solutions

Answer 299302 by richard1234(5390) About Me  on 2011-04-04 18:43:22 (Show Source):
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If -2 and 5 are roots, then x-(-2) and x-5 are factors of the polynomial. We can write the equation in the form

C(x+2)(x-5) = 0, where C is some nonzero constant.


Polynomials-and-rational-expressions/431173: is y to the 3 over 12x a polynomial
1 solutions

Answer 299301 by richard1234(5390) About Me  on 2011-04-04 18:40:01 (Show Source):
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y%5E3%2F12x is not a polynomial, because this the exponent of x is negative.


Trigonometry-basics/431008: how do i evaluate arccos(cos 7pi/2) using the properties of inverse functions?
1 solutions

Answer 299299 by richard1234(5390) About Me  on 2011-04-04 18:34:33 (Show Source):
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Arccosine and cosine are inverses of each other, so arccos%28cos%287pi%2F2%29%29 would be equal to 7pi%2F2. However, since the arccosine function restricts the range (since an infinite number of values are obtained), we say that this arccos%28cos%287pi%2F2%29%29+=+pi%2F2. Depending on the context of the problem, either answer could be correct, or even some general answer in the form pi%2F2+%2B+2k%2Api.


Quadratic_Equations/431169: What is x with x2-2x-13=0? And x2+12x-64=0? I've tried several times to find out what x is but neither of my solutions to these problems seem correct.
1 solutions

Answer 299298 by richard1234(5390) About Me  on 2011-04-04 18:31:48 (Show Source):
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Use the quadratic formula. The quadratic formula says that the roots of any quadratic equation in the form ax%5E2+%2B+bx+%2B+c+=+0 (with a,b,c real) are x+=+%28-b+%2B-+sqrt%28b%5E2+-+4ac%29%29%2F2a. From here, it's just simply identifying what a,b,c are and then substitution.


Matrices-and-determiminant/431116: The vertices of a triangle are given below. Use a determinant and the vertices of the triangle to find the area of the triangle.
(-3,-7.5), (-1.5,6.5), (1.5,-5.5)
** By the way: I know that the answer is 30, I just don't understand how to get the answer. Thank you
1 solutions

Answer 299297 by richard1234(5390) About Me  on 2011-04-04 18:21:54 (Show Source):
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We have Area+=+%281%2F2%29%28abs%28det%28A%29%29%29 where A =
| -3 -7.5 1 |
| -1.5 6.5 1 |
| 1.5 -5.5 1 | (3x3 matrix)

Here,

det%28A%29+=+-36+-+22.5+-+1.5+=+-60, abs%28det%28A%29%29+=+60

Therefore, the area is equal to 1/2 of 60, or 30.


Linear-equations/431130: y+4x=9 and x= y-4 I need to graph these on the computer any ideas ?
1 solutions

Answer 299295 by richard1234(5390) About Me  on 2011-04-04 18:07:02 (Show Source):
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You could go to WolframAlpha (www.wolframalpha.com) and provide your graphs there.


Evaluation_Word_Problems/431141: A restaurant is open from 2 pm to 2 am on Saturday and a maximum of 80 people are allowed inside. If f(t) is the number of people in the restaurant t hours after 2 pm on Saturday, what is a reasonable domain and range for y=f(t)?
Domain=
range=
1 solutions

Answer 299294 by richard1234(5390) About Me  on 2011-04-04 18:05:12 (Show Source):
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The restaurant is open up to 12 hours after the opening, so it is reasonable to say 0+%3C=+t+%3C=+12. Also, between 0 and 80 people are allowed, so 0+%3C=+f%28t%29+%3C=+80.


Trigonometry-basics/430780: Please any answer is appreciated for the following:
Prove that cos[3*pi/2 + x] = sinx
THank You
1 solutions

Answer 299110 by richard1234(5390) About Me  on 2011-04-03 21:27:47 (Show Source):
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Using the addition formula for cosine:

cos%283pi%2F2+%2B+x%29+=+cos%283pi%2F2%29cos%28x%29+-+sin%283pi%2F2%29sin%28x%29 Since cos%283pi%2F2%29+=+0 and sin%283pi%2F2%29+=+-1,

cos%283pi%2F2+%2B+x%29+=+0%2Acos%28x%29+-+%28-1%29%28sin%28x%29%29

= sin%28x%29.


Triangles/430574: Is a triangle with side lengths of 3,4, and 5 and another triangle with side lengths of 6, 8, and 11 a counterexample of two similar right triangles?
1 solutions

Answer 299105 by richard1234(5390) About Me  on 2011-04-03 21:19:44 (Show Source):
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No, because a triangle with sides 6,8,11 is not a valid counterexample (it is not a right triangle).