The coefficient of xr in the expansion of S=(x+3)n−1+(x+3)n−2(x+2)+(x+3)n−3(x+2)2+ …+(x+2)n−1 is
3n−r−2n−r
nCr3r−2r
nCr3n−r−2n−r
none of these
We can write S=(x+3)n−11−x+2x+3n1−x+2x+3=(3+x)n−(2+x)n
∴ Coefficient of xr=nCr3n−r−2n−r