In the expansion of x3−1x215, the constant term equals

Tr+1, the (r+1)th term is the expansion of x3−1x215is Tr+1=15Crx315−r−1x2r=15Cr(−1)rx45−5rTo obtain constant term, we set 45−5r=0⇒r=9∴  Coefficient of constant term is  15C9(−1)9=−15C9.