Geometry
# Sum and Difference Trigonometric Formulas

Using the cosine difference formula, what is the value of

$\cos 15 ^ \circ ?$

Which of the following is equal to

$\cos 56 ^ \circ \cos 11 ^ \circ + \sin 56 ^ \circ \sin 11 ^ \circ ?$

Which of the following is equal to

$\cos 15 ^ \circ \cos 45 ^ \circ - \sin 15 ^ \circ \sin 45 ^ \circ ?$

Which of the following is NOT equal to $\cos 2 \theta$?

$N$ and $M$ are real numbers such that

$\cos ( \alpha + \beta) = N \cos \alpha \cos \beta + M \sin \alpha \sin \beta$

is an algebraic identity. What is the value of $N+M$.