Question : If $a+\frac{1}{a}=3$, then $a^3+\frac{1}{a^3}$ is:
Option 1: 27
Option 2: 24
Option 3: 18
Option 4: 25
Correct Answer: 18
Solution :
Given: $(a + \frac{1}{a}) = 3$ (equation 1)
The value of $(a^3 + \frac{1}{a^3})$ by using the identity, $[a^3 + \frac{1}{a^3} = (a + \frac{1}{a})^3 – 3(a + \frac{1}{a})]$.
Substitute the value from the equation 1 into the above equation, we get,
$a^3 + \frac{1}{a^3} = (3)^3 -3(3)$
= 27 – 9 = 18
Hence, the correct answer is 18.
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