If a3 + b3 = 35 and ab = 6, find the value of a + b.
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 5.
a3 + b3 = (a + b)3 – 3ab(a + b)
⇒ 35 = (a + b)3 – 3 × 6 × (a + b)
⇒ 35 = (a + b)3 – 18(a + b)
⇒ (a + b)3 – 18(a + b) – 35 = 0
If a + b = 5 then 53 - 18(5) - 35 = 125 - 90 - 35 = 0