2x² + 2y²= 2(x² + y²)= 2(x² + y² + 2xy - 2xy)= 2(x² + y² + 2xy) - 4xy= 2(x + y)² - 4xy= 2×7² - 4×2= 98 - 8= 90 (x - y)²= x² + y² - 2xy= x² + y² + 2xy - 4xy= (x + y)² - 4xy= 7² - 4×2= 49 - 8= 41
