原式=(a-1)/(a+1)(a-1)+2/(a+1)(a-1)
=(a-1+2)/(a+1)(a-1)
=(a+1)/(a+1)(a-1)
=1/(a-1)
原式=(x²+xy-xy+y²)/(x+y)(x-y)
=(x²+y²)/(x²-y²)