(x^2 - 4y^2) ÷ (x+2y)
=(x-2y)(x+2y)÷（x+2y)
=x-2y
(4a^2 - 4a +1)(4a^2 +4a +1)÷(4a^2 - 1)
=(2a-1)²(2a+1)²÷(2a-1)(2a+1)
=(2a-1)(2a+1)
=4a²-1