1减去（a分之a-1）除以《（a+2分之a）-（a平方+2a分之1）》
=1-(a-1)/a÷[a/(a+2)-1/(a²+2a)]
=1-(a-1)/a÷(a²-1)/a(a+2)
=1-(a+2)/(a+1)
=(a+1-a-2)/(a+1)
=-1/(a+1)