1/(x^2+x-2)+1/(x^2+7x+10)=21/[(x+2)(x-1)]+1/[(x+5)(x+2)]=21/(x-1)+1/(x+5)=2(x+2)2(x+2)/[(x-1)(x+5)=2(x+2)当x=-2时 分母为0，无意义，即x+2≠0则(x-1)(x+5)=1 x^2+4x-6=0 (x+2)=10 x=-2±√10
