Identity (mathematics) - Simple English Wikipedia, The Free ...

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Identity relation

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A common example of the first meaning is the trigonometric identity

sin 2 ⁡ θ + cos 2 ⁡ θ = 1 {\displaystyle \sin ^{2}\theta +\cos ^{2}\theta =1\,}

which is true for all real values of θ {\displaystyle \theta } (since the real numbers R {\displaystyle {\mathbb {R}}} are the domain of both sine and cosine), as opposed to

cos ⁡ θ = 1 , {\displaystyle \cos \theta =1,\,}

which is only true for certain values of θ {\displaystyle \theta } in a subset of the domain.

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