Prove that cos2x=2 cos^2 x-1
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Answer:
Use the identity: cos (a + b) = cos a.cos b - sin a.sin b
cos
2
x
=
cos
(
x
+
x
)
=
cos
x
.
cos
x
−
sin
x
.
sin
x
=
cos
2
x
−
sin
2
x
=
=
cos
2
x
−
(
1
−
cos
2
x
)
=
2
cos
2
x
−
1
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