x-3xy-Y分之3x+5xy-3y=
=(3x+5xy-3y)/(x-3xy-Y)
=( 5+3*(1/y-1/x) )/ ( -3+ (1/y-1/x) ).分子分母同时除以xy
=(5-21)/(-3-7).将x分之一减y分之一等于7代入得
=(-16)/(-10)
=8/5
=1.6