If coefficient of (2r+3)th and (r−1)th terms in the expansion of (1+x)15 are equal, then value of r is
15C2r+2 = 15Cr−2
But 15C2r+2 = 15C15−(2r+2) = 15C13−2r
⇒ 15C13−2r = 15Cr−2 ⇒ r = 5.
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