If x4 - 79x2 + 1 = 0, then a value of x + x-1 can be :
Answer & explanation
Correct answer: option 1
If x4 - 79x2 + 1 = 0
If x4 -1 = 79x2
divide by x2 on both sides =
If x2 - \(\frac{1}{x^2}\) = n
Then, x + \(\frac{1}{x}\) = \(\sqrt {n + 2}\)
x2 - \(\frac{1}{x^2}\) = 79
x + \(\frac{1}{x}\) = \(\sqrt {79 + 2}\) = 9