(5x²+2y²)² - (5x²+2y²)(5x²-2y²)=(5x²+2y²)(5x²+2y²-5x²+2y²)=(5x²+2y²)·4y²=20x²y²+8y⁴
25x^4+2*(5x^2)*(2y^2)+4y^2-(25x^4-4y^2)=25x^4+20x^2 y^2 +4y^4-25x^4+4y^4=20x^2y^2+8y^4