4/x-2+3/2-x+1/x^2-x-2
=4/(x-2)-3/(x-2)+1/(x-2)(x+1)
=1/(x-2)+1/(x-2)(x+1)
=[(x+1)+1]/(x-2)(x+1)
=(x+2)/(x²-x-2)
第2题12/x^2-9+2/3-x
=12/(x+3)(x-3)-2/(x-3)
=[12-2(x+3)]/(x+3)(x-3)
=-2(x-3)/(x+3)(x-3)
=-2/(x+3)
第3题1/2a-1/a+b·(a+b/2a-2a^2+2ab/2a)
第4题1/x+1-x+3/x^2-1·x^2-2x+1/x^2+4x+3
第5题a+2-4/2-a