(x+3)(x-3)-x(x-2),=X^2-9-X^2+2X=2X-9x=4代入原式=2*4-9=-1
解x*x-9-x*x+2x=2x-9其中x=4，那么2x-9=2*4-9=-1故(x+3)(x-3)-x(x-2)=-1
