Reduction formulas

Idea of reduction

Often an angle is rewritten through an acute

\alpha\in\left(0;\dfrac{\pi}{2}\right)

. Multiples of

\dfrac{\pi}{2}

and

\pi

in the argument tell you which trig function you land on and what sign to put in front. Mnemonic: “cosines behave evenly”; “add

\dfrac{\pi}{2}

and sines swap with cosines”.

Before the final answer, check the sign from the quadrant of the actual angle

\theta

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