〔(x+2y)^2-(x+y)(3x-y)-5y^2〕÷2x
=(x^2+4y^2+4xy-3x^2+y^2-2xy-5y^2)÷2x
=(-2x^2+2xy)/2x
=2x(y-x)/2x
=y-x
=1/2-(-2)
=2.5