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# Sum and Difference Trigonometric Formulas

These formulas explain how to add and subtract trigonometric functions (and their arguments). If you've got sum time, see what a difference these formulas will make for your trig toolkit.

# Cosine - Sum and Difference 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 $$N+M$$.

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