Coefficeint of x40 in the expansion (1+x2)40(x2+2+1x2)−5 is :
30C15
1
30C20
None
(1+x2)40(x2+2+1x2)−5=(1+x2)40((x2+1)2x2)−5=(1+x2)40x10(x2+1)10
= x10(1+x2)30
The coefficient of xn in the expansion of (1+2x+3x2+...)1/2 is equal to: