Question

2x^2+y^2+5 xy+4y+5y+2

若x^2-y^2=0,求分式x^2-3xy+5y^2/3x^2-2xy+4y^2的值
要先化简再求值,不要把它直接 带进去
好的再加
快
由x^2-y^2=0
得x=±y
当x=y时
x^2-3xy+5y^2/3x^2-2xy+4y^2
=(x^2-3x^2+5x^2)/(3x^2-2x^2+4x)^2
=3/5
当x=-y时
x^2-3xy+5y^2/3x^2-2xy+4y^2
=(x^2+3x^2+5x^2)/(3x^2+2x^2+4x)^2
=1 这样回答老师说是 错的，分式可以化简

Answer 1

由x^2-y^2=0
得x=±y
当x=y时
x^2-3xy+5y^2/3x^2-2xy+4y^2
=(x^2-3x^2+5x^2)/(3x^2-2x^2+4x)^2
=3/5
当x=-y时
x^2-3xy+5y^2/3x^2-2xy+4y^2
=(x^2+3x^2+5x^2)/(3x^2+2x^2+4x)^2
=1