Use an Addition or Subtraction Formula to Write $\cos 75 ^ { \circ } \cos 15 ^ { \circ } + \sin 75 ^ { \circ } \sin 15 ^ { \circ }$

question 77

Essay

Use an addition or subtraction formula to write $\cos 75 ^ { \circ } \cos 15 ^ { \circ } + \sin 75 ^ { \circ } \sin 15 ^ { \circ }$ and find its exact value.

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