Question

Cos(81+a)=sin(k/3-a) find k

Answer 1

$\cos(81 + a) = \sin( \frac{k}{ 3} - a) \\ or \: \: \sin(90 - (81 + a)) = \sin( \frac{k}{3} - a ) \\ or \: \: 90 - 81 - a = \frac{k}{3} - a \\ or \: \: 9 = \frac{k}{3 } \\ or \: \: k = 27 \: \: \: \: \: \: \: (ans)$