16(2x-y)^2-(x+y)^2=[4(2x-y)]^2 - (x+y)^2=[4(2x-y) + (x+y)]*[4(2x-y) - (x+y)]=(9x-3y)(7x-5y)=3(3x-y)(7x-5y)999.5^2-0.5^2=(999.5+0.5)*(999.5-0.5)=1000*999=999000