(1) (1/2x+2/3y)(-2/3y+1/2x)
=1/4x²-4/9y²
(2) (-1-2a)(2a-1)
=1-4a²
(3) (2a+1)(4a^2)(2a-1)
=(4a^2)(4a²-1)=16a⁴-4a²