[﹙x-y﹚²+﹙x+y﹚﹙x-y﹚]÷2x²
=[x²-2xy+y²+x²-y²]÷2x²
=[2x²-2xy]÷2x²
=2x(x-y)/(2x*x)
=(x-y)/x
=1-y/x