Question

Найдите произведение ab, если a³+b³=13 и a+b=5

Answer 1

a + b = 5   ⇔  a² + b² = 5 - 2ab

a³ + b³ = (a+b) * (a² + b² - ab) = 5 * (5 - 3ab) = 13

25 - 15ab = 13

-15ab = -12

ab = 12/15 = 4/5