（x的平方-x）分之2+（x的平方+x）分之3=（x的平方-1）分之4
2/(x^2-x)+3/(x^2+x)=4/(x^2-1)
2/x(x-1)+3/x(x+1)=4/(x+1)(x-1)
两边同乘以x(x+1)(x-1)
2(x+1)+3(x-1)=4x
2x+2+3x-3=4x
x=1