4a^2b^2-(a^2+b^2)^2=
4aˇ2bˇ2 - (aˇ4+2.aˇ2bˇ2 + bˇ4) 4aˇ2bˇ2 -aˇ4 -2aˇ2bˇ2-bˇ4 = =2aˇbˇ2 -aˇ4 -bˇ4 = -(aˇ4 -2aˇ2bˇ2 +bˇ4) = =-(aˇ2-bˇ2)ˇ2