A mathematical relation of two trigonometric functions whose angles are complementary is called cofunction identity.
Co-function identities can be called as complementary angle identities and also called as trigonometric ratios of complementary angles. There are six trigonometric ratios of complementary angle identities in trigonometry.
Remember, theta ($\theta$) and $x$ represent angle of right triangle in degrees and radians respectively. You can use any one of them as formula in trigonometry problems.
The sine of complementary angle is equal to cosine of angle.
$\sin{(90^\circ-\theta)} \,=\, \cos{\theta}$
$\sin{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \cos{x}$
The cosine of complementary angle is equal to sine of angle.
$\cos{(90^\circ-\theta)} \,=\, \sin{\theta}$
$\cos{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \sin{x}$
The tangent of complementary angle is equal to cotangent of angle.
$\tan{(90^\circ-\theta)} \,=\, \cot{\theta}$
$\tan{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \cot{x}$
The cotangent of complementary angle is equal to tangent of angle.
$\cot{(90^\circ-\theta)} \,=\, \tan{\theta}$
$\cot{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \tan{x}$
The secant of complementary angle is equal to cosecant of angle.
$\sec{(90^\circ-\theta)} \,=\, \csc{\theta}$
$\sec{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \csc{x}$
The cosecant of complementary angle is equal to secant of angle.
$\csc{(90^\circ-\theta)} \,=\, \sec{\theta}$
$\csc{\Big(\dfrac{\pi}{2}-x\Big)} \,=\, \sec{x}$
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