Question

If a+b=9 and ab=20, then find the value of a3+b3?

Answer 1

Answer:

Step-by-step explanation:

a+b=9

ab=20

(a+b)³=a³+b³+3ab(a+b)

9³=a³+b³+3*20*9

729=a³+b³+540

a³+b³=729-540

        =189

Answer 2

Step-by-step explanation:

a + b = 9

ab = 20

a² + b² = (a + b)² - 2ab

Substituting, we get,

a² + b² = (9)² - 2(20)

=> a² + b² = 81 - 40 = 41

Now,

a³ + b³ = (a + b)(a² - ab + b²)

Substituting, we get,

a³ + b³ = 9 × (41 - 20)

=> a³ + b³ = 9 × 21

=> a³ + b³ = 189