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Section A.4 Trigonometric Identities

Pythagorean Identities.

The following trigonometric identities are useful in solving differential equations:

: $\sin^{2} (θ) + \cos^{2} (θ) = 1$

$1 + \tan^{2} (θ) = \sec^{2} (θ)$

$1 + \cot^{2} (θ) = \csc^{2} (θ)$

Even and Odd Properties.

: $\sin (- θ) = - \sin (θ)$
: $\cos (- θ) = \cos (θ)$
: $\tan (- θ) = - \tan (θ)$
: $\csc (- θ) = - \csc (θ)$
: $\sec (- θ) = \sec (θ)$
: $\cot (- θ) = - \cot (θ)$

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