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Prove that
Solution:
In proving the trigonometric functions, the first thing that you have to do is to choose the more complicated side of the equation and then simplify and compare with the other side of the equation if they are equal or not.
In this case, let's choose the left side of the equation as follows
If you will continue to expand further the above equation, it will be more complicated and longer. Let's substitute all trigonometric functions with another variables as follows
Let
then the above equation becomes
since
then the above equation becomes
Therefore,
