Given expression is sin(45)sin(15)
this expression best matches with left side of the formula:
2 sin(A) sin(B)= cos(A-B) - cos(A+B)
so we can plug given angles 45 and 15 there
2 sin(45) sin(15)= cos(45-15) - cos(45+15)
2 sin(45) sin(15)= cos(30) - cos(60)
sin(45) sin(15)= [cos(30) - cos(60)]/2
We are getting negative sign and cos in the solution while none of the given choices have same situation so answer will be none of them.
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For cos(75)-cos(15), we will use formula:
Now plug the given angles
Hence choice B is correct.