42² = 40² + 2 * 40 * 2 + 2² = 1600 + 160 + 4 = 1764

99² = (100 - 1)² = 100² - 2 * 100 * 1 + 1 = 9801

47² - 37² = (47 - 37)(47 + 37) = 10 * 84 = 840

22 * 18 = (20+2)(20-2) = 20² - 2² = 400 - 4 = 396