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Mathematics
Problem
In Exercises 11-51, prove the identity. 15.
(
1
−
cos
u
)
(
1
+
cos
u
)
cos
2
u
=
tan
2
u
\frac{(1-\cos u)(1+\cos u)}{\cos ^{2} u}=\tan ^{2} u
c
o
s
2
u
(
1
−
c
o
s
u
)
(
1
+
c
o
s
u
)
=
tan
2
u
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