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Find the third term in the expansion of (ab)6 in a...

Find the third term in the expansion of (ab)6 in ascending powers of b.

A.

15a4b2

B.

15a4b2

C.

15a3b3

D.

15a3b3

Correct answer is B

(ab)6=6C0(a)6(b)0+6C1(a)5(b)1+6C2(a)4(b)2+...

Third term = 6C2(a)4(b)2=6!(62)!2!(a4)(b2)

= 15a4b2