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Can someone help me to solve this problem?

Find all number pairs $x,y$ that satisfy the equation:

$$\tan^4(x) + \tan^4(y) + 2\cot^2(x)\cot^2(y) = 3 + \sin^2(x+y)$$

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1 Answer 1

By AM-GM inequality: $$(\tan x)^4 + (\tan y)^4 + (\cot x)^2\cdot (\cot y)^2 + (\cot x)^2\cdot (\cot y)^2 \geq 4 \geq 3 + (\sin(x+y))^2 $$ So the equation occurs when: $\sin(x+y) = 1, -1$, and $\tan x = \tan y, -\tan y$, and $\tan x = \cot x, -\cot x$. So $\tan x = 1, -1 = \tan y$. You can look at cases.

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Can u please the am gm part in more detail – user34304 Apr 17 '14 at 4:27

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