# Trigonometric Identity

We know that $$\cos { 2x } =\cos ^{ 2 }{ x } -\sin ^{ 2 }{ x }$$

This is just a proof I have made. I don't know if it already exists but I just wanted to share it with you.

${ \left( { e }^{ ix } \right) }^{ 2 }={ e }^{ 2ix }\\ \\ { \left( { e }^{ ix } \right) }^{ 2 }={ e }^{ i\times 2x }\\ \\ { \left( \cos { x } +i\sin { x } \right) }^{ 2 }=cos2x+isin2x\\ \\ \cos ^{ 2 }{ x } +{ i }^{ 2 }\sin ^{ 2 }{ x } +2i\cos { x } \sin { x } =cos2x+isin2x\\ \\ \cos ^{ 2 }{ x } +\left( -1 \right) \sin ^{ 2 }{ x } +i\times 2\cos { x } \sin { x } =cos2x+isin2x\\ \\ \cos ^{ 2 }{ x } -\sin ^{ 2 }{ x } +i\sin { 2x } =cos2x+isin2x\\ \\ So,\quad \cos { 2x } =\cos ^{ 2 }{ x } -\sin ^{ 2 }{ x }$

Note by Archit Boobna
6 years, 2 months ago

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This also exists !!

- 6 years, 2 months ago

But I thought it myself.

- 6 years, 2 months ago

Ok ! Did you try all of my problems - new ones ?

- 6 years, 2 months ago

Nice observational skill Archit :)

- 6 years, 2 months ago

Thanks!

- 6 years, 2 months ago