(x+2y)(x-2y)(x^2-4y^2)
=(x+2y)(x-2y)(x+2y)(x-2y)
=[(x+2y)(x-2y)]²
x=5/3 y=2/3
x+2y=5/3+4/3=9/3=3
x-2y=5/3-4/3=1/3
(x+2y)(x-2y)=3×1/3=1
代入得
=1²
=1