y/(x＋y)＋(x²－xy)/(x²－y²)
＝y/(x＋y)＋x(x－y)/[(x－y)(x＋y)]
＝y/(x＋y)＋x/(x＋y)
＝(x＋y)/(x＋y)
＝1
(x－4/x)÷(x－2)/x
＝(x²/x－4/x)×x/(x－2)
＝(x²－4)/x×x/(x－2)
＝[(x＋2)(x－2)]/x×x/(x－2)
＝x＋2