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
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