a²+2a-15=0
a²+2a+1=16
(a+1)²=16
原式=1/(a+1) -(a+2)/(a+1)(a-1) ×(a-1)²/(a+1)(a+2)
=1/(a+1)-(a-1)/(a+1)²
=(a+1-a+1)/(a+1)²
=2/(a+1)²
=2/16
=1/8