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 4/1_数学解答

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

人气：199 ℃　时间：2020-08-29 03:37:00

解答

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