cos(x+3π)−sin(x+6π)
cos(x+3π)=cosxcos3π−sinxsin3π
sin(x+6π)=sinxcos6π+cosxsin6π
cos(x+3π)−sin(x+6π)= cosxcos3π−sinxsin3π −sinxcos6π−cosxsin6π
therefore
cosxcos3π−sinxsin3π−sinxcos6π−cosxsin6π
this becomes
21cosx−21cosx−23sinx−23sinx
0−3sinx=−3sinx
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