Coefficient of y3.x6 in the binomial expansion (x+2y)9
276
672
762
267
Tr+1 = nCrxn−ryr = 9Crx9−r(2y)r
r = 3 ⇒ coeff = 9C323 = 672
Prove that the coefficient of xn in the binomial expansion of (1+x)2n is twice the coefficient of xn in the binomial expansion of (1+x)2n−1.