(p+q)^2 (p-q)^2 =( (p+q)* (p-q) )² = (p²-q²)² = p⁴-2p²q²+q⁴ (a+b)^2-(a-b)^2 = a²+2ab+b² - (a²-2ab+b²) = a²+2ab+b² - a²+2ab-b² = 4ab