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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.

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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