If x - 1/x = 5 √21 then find the value of ( x + 1/x )2

⇒ If x - \(\frac{1}{x}\) = a then x + \(\frac{1}{x}\) = \(\sqrt {a^2 + 4}\)

Here,

x - \(\frac{1}{x}\) = 5\(\sqrt{21}\), then 

⇒ x + \(\frac{1}{x}\) = \(\sqrt {(5\sqrt{21})^2 + 4}\) = 23

⇒ ( x + \(\frac{1}{x}\) )² = 23² = 529