We have to show that cos 3B + cos B = 2(cos 2B)* cos
B
We use the relations: cos 2B = 2*(cos B)^2 - 1 and cos 3B
= 4(cos B)^3 - 3cos B
We start with the left hand
side
cos 3B + cos B
=>
4(cos B)^3 - 3cos B + cos B
=> 4(cos B)^3 - 2*cos
B
=> 2* cos B ( 2* ( cos b)^2 -
1)
=> 2 * cos B * cos
2B
which is the right hand
side.
The required relation cos 3B + cos B =
2(cos 2B)* cos B is proved
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