1减a分之a-1 除以 a的平方+2a分之a的平方-1
=1-(a-1)/a÷(a²-1)/(a²+2a)
=1-(a-1)/a×[a(a+2)/(a-1)(a+1)]
=1-(a+2)/(a-1)
=(a-1-a-2)/(a-1)
=-3/(a-1)