1/x-1/y=5
1/y-1/x=-5
(3x+5xy-3y)/(x-3xy-y)(分子分母同时除以xy)
=(3x/xy+5xy/xy-3y/xy)/(x/xy-3xy/xy-y/xy)
=(3/y+5-3/x)/(1/y-3-1/x)
=(3/y-3/x+5)/(1/y-1/x-3)
=(-5*3+5)/(-5-3)
=-10/(-8)
=5/4
x²+1/x²
=(x+1/x)²-2
=3²-2
=9-2
=7