(a - b)² = a² - 2ab + b²
a² - b² = (a - b)(a + b)
999² = (1000 - 1)² = 1000000 - 2*1000*1 + 1 = 1000001 - 2000 = 998001
44*36 = (40 + 4)(40 - 4) = 40² - 4² = 1600 - 16 = 1584
