原式=x²-4x+4/x²-4 - x+2/x-2
=(x-2)^2/(x+2)(x-2)-(x+2)/(x-2)
=(x-2)/(x+2)-(x+2)/(x-2)
=-8x/(x+2)(x-2)
=-8x/(x^2-4)
把x=根下2代入
原式=4√2