If the coefficient of (2r+4)th and (r−2)th terms in the expansion

Question -

If the coefficient of $(2 r + 4)^{t h}$ and $(r - 2)^{t h}$ terms in the expansion of ( $1 + x)^{18}$ are equal, then r =

Options:

A . 12

B . 10

C . 8

D . 6

Answer: Option D
:
D

$^{18} C_{2 r + 3}$ = $^{18} C_{r - 3}$ ⇒ 2r + 3 + r - 3 = 18 ⇒ r = 6

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