Which expression is equivalent to cos(40°)cos(10°) + sin(40°)sin(10°)?
Answer
A
sin(40° – 10°)
B
cos(40° + 10°)
C
sin(40° + 10°)
D
cos(40° – 10°)
Explanation
The cosine difference identity is cos(a − b) = cos(a)cos(b) + sin(a)sin(b). With a = 40° and b = 10°, the expression becomes cos(40° − 10°), which is choice D.
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