左边=[(x^2+4x+3）-（x^2-2x）]/(x-2)(x-3)=(6x+3)/(x-2)(x+3)所以6x+3=a,又因x为变量，a为定值，所以x=0，a=3
(x+1)/(x-2)- x/(x+3)=(x+a)/(x-2)(x+3)(x+3)(x+1) - x(x-2) = x+aput x=03 = a
