[(a+1/2b)^2+(a-1/2b)^2](2a^2-1/2b^2)
=[a^2+ab+1/4b^2+a^2-ab+1/4b^2](2a^2-1/2b^2)
=(2a^2+1/2b^2)(2a^2-1/2b^2)
=4a^4-1/4b^4