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Find the coefficient of x2in the binomial expansion of ...

Find the coefficient of x2in the binomial expansion of (x+2x2)5

A.

10

B.

40

C.

32

D.

80

Correct answer is A

(x+2x2)5

n = 5,  r = 4,  p = x  and q = 2x2  

5C4x4 (2x2)1 = 5C4 2x4x2 

5C4 2x2 = 5![54]!4! * 2x2

54!4!2x2 = 5 * 2x2 = 10x2

The coefficient is 10.