12. Trigonometric Identities

Find the exact value of the expression.

$\cos105\degree$

Find the exact value of the expression.

$\sin15\degree$

Find the exact value of the expression.

$\cos\frac{5\pi}{12}$

Find the exact value of the expression.

$\cos80\degree\cos20\degree+\sin80\degree\sin20\degree$