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Question

If the coefficients of (2r+4)th,(r2)th terms in the expansion of (1+x)18 are equal, find r.

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Solution

A] Given (1+x)18
(2r+3)+1=18C2r+3(x)2r+3
T(r3)+1=18C2r3(x)r3
So,
18C2r+3(x)2r+3=18Cr3(x)r3 [nCx=ncyx+y=n]
= 2r + 3 + r - 3 = 18
= 3r = 18
r=6

1192392_1294261_ans_401d76bf78b6473da7d98c4b00fe69d0.jpg

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