Aasav's square roots

Algebra Level 2

If x = 2 + 2 x= \sqrt{2+\sqrt{2}} , what is x 4 + 4 x 4 ? x^4+\frac{4}{x^4}?

This problem is posed by Aasav B .


The answer is 12.

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3 solutions

Thomas Lee
Jun 30, 2015
  1. Let x 4 + 4 x 4 = y x^4+\frac{4}{x^4}=y
  2. x 4 + 4 + 4 x 4 = y + 4 x^4+4+\frac{4}{x^4}=y+4
  3. ( x 2 + 2 x 2 ) 2 = y + 4 (x^2+\frac{2}{x^2})^2=y+4
  4. ( ( 2 + 2 ) 2 + 2 ( 2 + 2 ) 2 ) 2 = y + 4 ((\sqrt { 2+\sqrt { 2 } } )^2+\frac{2}{(\sqrt { 2+\sqrt { 2 } } )^2})^2=y+4
  5. ( 2 + 2 + 2 2 + 2 ) 2 = y + 4 (2+\sqrt{2}+\frac{2}{2+\sqrt{2} })^2=y+4
  6. ( 2 + 2 + 2 2 + 2 2 2 2 2 ) 2 = y + 4 (2+\sqrt{2}+\frac{2}{2+\sqrt{2}}*\frac{2-\sqrt{2}}{2-\sqrt{2}})^2=y+4
  7. ( 2 + 2 + 2 ( 2 2 ) 2 2 ( 2 ) 2 ) 2 = y + 4 (2+\sqrt{2}+\frac{2(2-\sqrt{2})}{2^2-(\sqrt{2})^2})^2=y+4
  8. ( 2 + 2 + 2 ( 2 2 ) 2 ) 2 = y + 4 (2+\sqrt{2}+\frac{2(2-\sqrt{2})}{2})^2=y+4
  9. ( 2 + 2 + 2 2 ) 2 = y + 4 (2+\sqrt{2}+2-\sqrt{2})^2=y+4
  10. 4 2 = y + 4 4^2=y+4
  11. y = 12 y=12
Ariijit Dey
Jun 29, 2014

Differentiate the function & get the max. value; Apply the periodicity of the values of X & then solve.

Ram Meena
Jun 14, 2014

answer will be (X^2 + (2/X^2)) - 4 x^2=2+root2 1/x^2=2-root2 just manipualte and get 12

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