若x/y=2/7 (/是分数线)
问题描述：
若x/y=2/7 (/是分数线)
则 x^2-3xy+2y^2/2x^2-3xy+7y^2的值是 ( )
A 28/103 B 4/103 C 20/103 D7/103
^是乘方
答
x/y=2/7
x=(2/7)y
(x^2-3xy+2y^2)/(2x^2-3xy+7y^2)
=[(4/49)y^2-(6/7)y^2+2y^2]/[(8/49)y^2-(6/7)y^2+7y^2]
=(60/49)y^2/[(309/49)y^2]
=20/103