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

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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