(3x-y)(y+3x)-(4x-3y)(4x+3y)
=-(y-3x)(y+3x)-(4x-3y)(4x+3y)
=-y^2+9x^2-16x^2+9y^2
=8y^2-7x^2
因为x=1;y=1/2
所以原式=8*(1/4)-7*1=2-7=-5