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How high should I know my factorials by memory? I've had

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Joined: 31 Jul 2008
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How high should I know my factorials by memory? I've had [#permalink]

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New post 31 Jul 2008, 13:18
How high should I know my factorials by memory? I've had problems where there's a 12! but that seems crazy to have to memorize!

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Re: Factorials [#permalink]

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New post 31 Jul 2008, 13:19
The GMAT will require factorials in the context of Combination and Permutation problems. The way those formulas are set up to divide factorials before you actually calculate them.

For example:

8_C_3 = 8!/ [3!(8-3)!] = 8*7*6*5*4*3*2*1 / 3*2*1 * 5*4*3*2*1

Here the 5 through 1 factors in the numerator and denominator cancel to 1 leaving:

8*7*6 / 3*2*1

and the 3*2 in the denominator cancel the 6 factor in the numerator. That leaves simply 8*7, or 56.

I have lots of students who want to calculate 8! then divide it by 5! and 3! but canceling common factors in the numerator and denominator is WAY faster and less prone to error.

This is the manner in which factorials are tested on the GMAT.
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Re: Factorials [#permalink]

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New post 31 Jul 2008, 13:21
I think in working the factorials in prep for my GMAT i got to the point where I remembered that 7! is 5040. I never needed to know. Most of the time when you're dealing with factorials, it's a Comb/Perm problem and you can cancel many of the parts out. You'd want to write it on your paper like this in order to cancel out the top and bottom where you can

\(C_7^3 = \frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1} = \frac{7*5}{1}=35\)

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Re: Factorials   [#permalink] 31 Jul 2008, 13:21
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