若1/x+1/y=3,则2x-xy+2y/x-2xy+y的值是多少?还有已知x/y=2/7,求x²-3xy+2y²/2x²-3xy+2y&s

1/x+1/y=3(x+y)/xy=3x+y=3xy(2x-xy+2y)/(x-2xy+y)=(2x+2y-xy)/(x+y-2xy)=(2*3xy-xy)/(3xy-2xy)=5xy/xy=5(x²-3xy+2y²)/(2x²-3xy+2y²)分子分母同时除以y^2=(x^2/y^2-3x/y+2)/(2x^2/y^2-3x/y+2)=(4...