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( n ) a^(n-(r-1)) * b^(r-1)
r-1
I don't understand what the (n & r-1) terms in the parenthesis at the beginning of the expression are. From the solution, it suggests that the first term ends up resolving to a division of factorials such as:
(6! / ((6-3)! * 3!)) where n=6 and r=4...
I just dont understand how they got from the first form to the factorial form. 1 solutions
Answer 395467 by solver91311(17077) on 2012-07-13 16:15:20 (Show Source):
You can put this solution on YOUR website!
I think what you are looking at is (read: "n choose r - 1") which is the number of combinations of things taken at a time. Such as, how many ways can I choose from 10 different books on a shelf if I take them 3 at a time and I don't care what order the three are in when I get them in my hand?
In general, is calculated by . Hence, your is exactly correct given and .
Here's the logic: Let's say you have 6 things and want to choose 3 of them. There are 6 ways to choose the first one, then since you didn't replace the first one you chose, there are 5 ways to choose the second one for each one of the 6 ways to choose the first one. Then there are 4 ways to choose the third one for each of the 30 ways to pick the first two, which works out to which comes from . But that number is too large by a factor of the number of ways to arrange the three things in your hand, namely , and that is where the other denominator factor comes from.
You might want to compare this to the number of permutations of things taken at a time. With permutations, order matters. Such as you have 20 people in your club and you want to know how many different ways you can select a President, Secretary, and Treasurer. Here order matters because Suzy being the president is a different outcome than Suzy being the Secretary, for example. Permutations are calculated . See the difference?
By the way, this is also the th coefficient (counting from zero) of the binomial expansion of and the th element of the th row of Pascal's Triangle.
John

My calculator said it, I believe it, that settles it
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Travel_Word_Problems/628193: two airplanes leave the airport at the same time, one heading west the other east. their average speeds differ by 10 miles per hour. after 1.5 hours, they are 520 miles apart. what is the approximate average speed of each plane over the 105 hours? 1 solutions
Answer 395449 by solver91311(17077) on 2012-07-13 14:10:23 (Show Source):
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Exponents/628010: Use algebraic procedures to find the exact solution(s) of the equation. (Enter your answers as a comma-separated list. If there is no solution, enter NO SOLUTION.)
4^(4x − 3) = 5^(−x − 1)
x=(?,?) 1 solutions
Answer 395366 by solver91311(17077) on 2012-07-12 22:40:13 (Show Source):
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Geometry_Word_Problems/628074: A rectangular piece of metal is 30 in longer than it is wide. Squares with sides 6 in. long are cut from four corners and the flaps are folded upward to form on open box. If the volume of the box is 594 cubic inches, what were the original dimensions of the piece of metal? 1 solutions
Answer 395351 by solver91311(17077) on 2012-07-12 21:33:32 (Show Source):
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Equations/628054: How do you Use an algebraic approach to solve the problem.
A board 45 feet long is cut into two pieces such that the length of one piece is two-thirds of the length of the other piece. Find the length of the shorter piece of board.
so far what i have is 2/3x=45 is this right? if so how do you solve? 1 solutions
Answer 395337 by solver91311(17077) on 2012-07-12 20:42:16 (Show Source):
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�t�e�s�t/628013: Please help me with this question:
Suppose that y=5x+2 and we want y to be within 0.07 unit from 6.
for what values of x will this be true?
Thank you
Samantha 1 solutions
Answer 395325 by solver91311(17077) on 2012-07-12 19:59:22 (Show Source):
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test/628003: Find a polynomial f(x) with real coefficients having the given degree and zeros.
Degree 5; zeros: 6;-i;-7+i
f(x)=a?
Do not write the polynomial in factored form. Multiply it out. 1 solutions
Answer 395294 by solver91311(17077) on 2012-07-12 17:44:17 (Show Source):
You can put this solution on YOUR website!
If is a zero of a polynomial, then is a factor of the polynomial. Complex zeros of polynomials with real coefficients always appear in conjugate pairs. You didn't specify that your function has real coefficients, but unless that is true it is impossible to solve the problem as posed. Hence, I make the assumption of real coefficients.
is a real zero, so is a factor of the polynomial. The conjugate of a general complex number is formed by changing the sign on the second term thus: is a complex zero. To form the conjugate of , rewrite it as , then it should be clear that the conjugate is which can be written as simply . Likewise your other zero, , is a complex number, hence the fifth and last zero of your 5th degree polynomial is
Now just form the factors:
Now the only work left for you to do is to multiply out the five binomials. Note that the product of two conjugate binomials is the difference of two squares, but because , the product of two complex conjugates becomes the sum of two squares.
John

My calculator said it, I believe it, that settles it
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Triangles/627928: Which three lengths could be the sides of a triangle:
8,15,22
22,5,10
11,6,17
6,22,10 1 solutions
Answer 395219 by solver91311(17077) on 2012-07-12 12:04:05 (Show Source):
You can put this solution on YOUR website!
The sum of the measures of any two sides of a triangle must be strictly greater than the measure of the third side. Go through your list. For each of the sets of three numbers, find the two smallest. Add them. If the sum is less than or equal to the largest number, then the set of numbers does NOT represent the measures of the sides of a triangle.
John

My calculator said it, I believe it, that settles it
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