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Find the coefficient of x3y2 in the binomial ...

Find the coefficient of x3y2 in the binomial expansion of (x-2y)5

A.

-80

B.

10

C.

40

D.

90

Correct answer is C

x3y2 in (x-2y)5


n = 5, r = 3, p = x, q = -2y

 

5C3 * x3 -2y2

5C3 = 5![53]!3!

543!2!3! →  542

5C3 = 10

: 5C3 * x3 -2y2 = 10 *  x3 4y2

 40x3y2
the coefficient is 40