If the coefficient of $(2 r + 3)^{t h}$ and $(r - 3)^{t h}$ terms in the expansion of ( $1 + x)^{18}$ are equal, then r =

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Solution

The correct option is D
6

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

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