(x+y):y=8:3
3x+3y=8y
得3x=5y
设x=5a,y=3a
所以
(2x+5y)/(3x-y)
=(10a+15a)/(15a-3a)
=25a/12a
=25/12
(x^2-xy)/(y^2+3xy)
=(25a^2-15a^2)/(9a^2+45a^2)
=10a^2/54a^2
=5/27