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Mathematics
Mathematics
Problem
Prove that
cos
(
x
+
y
)
+
cos
(
x
−
y
)
=
2
cos
x
cos
y
\cos(x+y)+\cos(x-y)=2\cos x\cos y
cos
(
x
+
y
)
+
cos
(
x
−
y
)
=
2
cos
x
cos
y
.
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