Cosine of Complement equals Sine

Theorem

 * $\cos \left({\dfrac \pi 2 - \theta}\right) = \sin \theta$

where $\cos$ and $\sin$ are cosine and sine respectively.

That is, the sine of an angle is the cosine of its complement.

Also see

 * Sine of Complement equals Cosine
 * Tangent of Complement equals Cotangent
 * Cotangent of Complement equals Tangent
 * Secant of Complement equals Cosecant
 * Cosecant of Complement equals Secant

Historical Note
Discovered and documented by in the 6th century CE.