Verifying trig identities is important, especially when you're about to let one of them into the country—which is why they should keep their passports on them at all times.

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Verifying Trig Identities 
Up Next:
Tangent HalfAngle Formula 
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Special Right Triangles 
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SOHCAHTOA 
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Sines 
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Cosines 
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Law of Sines