解
1/(a+1)+(a²-a)/(a²-2a-3)÷a/(a-3)
=1/(a+1)+a(a-1)/(a-3)(a+1)×(a-3)/a
=1/(a+1)+(a-1)/(a+1)
=(1+a-1)/(a+1)
=a/(a+1)