x/y=2/7
(x^2-3xy+2y^2)/(2x^2-3xy+7y^2)分子分母同时除以y^2
=(x^2/y^2-3xy/y^2+2y^2/y^2)/(2x^2/y^2-3xy/y^2+7y^2/y^2)
=[(x/y)^2-3x/y+2]/[2(x/y)^2-3x/y+7]
=[(2/7)^2-3*2/7+2]/[2*(2/7)^2-3*2/7+7]
=(4/49-6/7+2)/(8/49-6/7+7)
=(4/49-42/49+98/49)/(8/49-42/49+343/49)
=(60/49)/(309/49)
=60/309
=20/103