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Find the 3rd term of (x21)8 in descending...

Find the 3rd term of (x21)8 in descending order of x.

A.

x78

B.

7x616

C.

7x54

D.

35x48

Correct answer is B

(x21)8=8C8(x2)8(1)0+8C7(x2)7(1)1+8C6(x2)6(1)2+...

The third term in the expansion =8C6(x2)6(1)2

= 8!6!2!(x664)(1)

= 28×x664=7x616