（3/a^2-4a-5）-（4/a^-2a-15）
=3/(a-5)(a+1)-4/(a-5)(a+3)
=[3(a+3)-4(a+1)]/(a+1)(a+3)(a-5)
=(3a+9-4a-4)/(a+1)(a+3)(a-5)
=-(a-5)/(a+1)(a+3)(a-5)
=-1/(a+1)(a+3)