(x-1/2y-1)(x-1/2y+1)-(x-1/2y-1)的2次
=(x-1)²/[(2y-1)(2y+1)]-(x-1)²/(2y-1)²
=(x-1)²/(2y-1)[1/(2y+1)-1/(2y-1)]
=(x-1)²/(2y-1)(2y-1-2y-1)/[(2y-1)(2y+1)]
=-2(x-1)²/[(2y-1)²(2y+1)]
(x-1/2y-1)(x-1/2y+1)-(x-1/2y-1)的2次
=(x-1)²/[(2y-1)(2y+1)]-(x-1)²/(2y-1)²
=(x-1)²/(2y-1)[1/(2y+1)-1/(2y-1)]
=(x-1)²/(2y-1)(2y-1-2y-1)/[(2y-1)(2y+1)]
=-2(x-1)²/[(2y-1)²(2y+1)]