Question

cos(180° – a)
sin (90° – a)

Answer 1

Формулы приведения.

$\cos(180 {}^{ \circ} - \alpha ) = - \cos \alpha . \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ \\ - - - - - - - - - - -$

$\sin(90 {}^{ \circ} - \alpha ) = \cos \alpha .$