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do you have a unit circle with you?

very good

those are 2 of infinitely many angle values x that make cos(x) = -1/2 true

yes it is

And could you help me with 1 more?

sure

#2 is incorrect
you use sin(A-B) = sin(A)cos(B) - cos(A)sin(B)

#3 is correct
cos(A-B) = cos(A)cos(B) + sin(A)sin(B)

So for#2, instead of sin9x, would it be sin7x?

yes

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