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Problem
## Miscellaneous Exercise on Chapter 2 Prove that 5.
cos
−
1
4
5
+
cos
−
1
12
13
=
cos
−
1
33
65
\cos ^{-1} \frac{4}{5}+\cos ^{-1} \frac{12}{13}=\cos ^{-1} \frac{33}{65}
cos
−
1
5
4
+
cos
−
1
13
12
=
cos
−
1
65
33
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