If x2 + 1/x2 = 7, then the value of x3 + 1/x3 where x > 0 is equal to:
Show answer & explanation
Correct answer: A
Explanation
The correct answer is option 1 i.e. 18
x2 + 1/x2 = 7
On adding 2 both sides we get
(x + 1/x)2= 9
x + 1/x = 3
On cubing both sides we get,
x3 + 1/x3 + 3(x + 1/x) = 27
x3 + 1/x3 = 27 - 9 = 18.