Question : If $a+\frac{1}{a}=7$, then $a^5+\frac{1}{a^5}$ is equal to:
Option 1: 15127
Option 2: 13127
Option 3: 14527
Option 4: 11512
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Correct Answer: 15127
Solution :
Given:
$a+\frac{1}{a}=7$----------------(1)
Squaring both sides we get,
⇒ $a^2+\frac{1}{a^2}+2×a×\frac{1}{a}=49$
⇒ $a^2+\frac{1}{a^2}=49-2=47$ ------------------(2)
Now cubing equation 1 we get,
⇒ $a^3+\frac{1}{a^3}+3a×\frac{1}{a}(a+\frac{1}{a})=7^3$
⇒ $a^3+\frac{1}{a^3}+3×7=343$
⇒ $a^3+\frac{1}{a^3}=343-21=322$ ----------------(3)
Multiplying equations 2 and 3 we get
⇒ $a^5+a+\frac{1}{a}+\frac{1}{a^5}=47×322$
⇒ $a^5+\frac{1}{a^5}=47×322-(a+\frac{1}{a})$
⇒ $a^5+\frac{1}{a^5}=47×322-7$
$\therefore a^5+\frac{1}{a^5}=15127$
Hence, the correct answer is 15127.
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