If x > 0 and $x^4 + \frac{1}{x^4} = 2207$, what is the value of $( x^5 + \frac{1}{x^5})$
Answer & explanation
Correct answer: option 1
We know that,
x5 + $\frac{1}{x^5}$ = (x2 + $\frac{1}{x^2}$) × (x3 + $\frac{1}{x^3}$) – (x + $\frac{1}{x}$)
If x4 + \(\frac{1}{x^4}\) = a
then x2 + \(\frac{1}{x^2}\) = \(\sqrt {a + 2}\) = b
and x + \(\frac{1}{x}\) = \(\sqrt {b + 2}\)
If x > 0
If $x^4 + \frac{1}{x^4} = 2207$
what is the value of $( x^5 + \frac{1}{x^5})$
If $x^4 + \frac{1}{x^4} = 2207$
then, x2 + \(\frac{1}{x^2}\) = \(\sqrt {2207 + 2}\) = 47
and x + \(\frac{1}{x}\) = \(\sqrt {47 + 2}\) = 7
We know that If x + \(\frac{1}{x}\) = n
then, $x^3 +\frac{1}{x^3}$ = n3 - 3 × n
$x^3 +\frac{1}{x^3}$ = 73 - 3 × 7 = 322
So,
x5 + $\frac{1}{x^5}$ = 47 × 322 – 7
x5 + $\frac{1}{x^5}$ = 15127