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# expansion c4/c2 watch

1. Theres another way of working out
4 nCr 2 besides using your calculator.. anyone know what it is?
i thought it involved factorials, but i think im just confusing myself.

basically i need another way of working out things like (3 nCr 5)?
2. You're given the formula in the exam, you should learn how to use it without nCr as you can't use it in the C4. Anyway, the formula is:

(a+b)^n = a^n + (a^n)(b) + n(n-1)/2! x a^n-2 x b^2 +n(n-1)(n-2)/3! x a^n-3 x b^3......
3. no i just need a way of working out the 4 nCr 2 bit.. i can do the rest of the expansion.
4. (Original post by cuddles x)
no i just need a way of working out the 4 nCr 2 bit.. i can do the rest of the expansion.

EDIT: Only for integer values of n & r, of course.
5. (Original post by james.h)

wicked thanks.

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