1/x-1/y=5
(y-x)/xy=5
(x-y)/xy=-5
(3x+5xy-3y)/(x-3xy-y)
=[3(x-y)+5xy]/[(x-y)-3xy]
=[3(x-y)/xy+5]/[(x-y)/xy-3]
=[3×(-5)+5]/(-5-3)
=(-10)/(-8)
=5/4
x+1/x=3
(x+1/x)²=9
x²+2+1/x²=9
x²+1/x²=7