Trigonometry question

[xraysp3x]

Can you help me please?

sin a - sin b =1/2 cos b - cos b = 3/2cos(a+b) = ?
I know this:cos(a+b) = cos a*cos b-sin a*sin b
But I can't figure out anything, please help.

 

[Deleted member 4993]

Can you help me please?

sin a - sin b =1/2 cos b - cos b = 3/2cos(a+b) = ?
I know this:cos(a+b) = cos a*cos b-sin a*sin b
But I can't figure out anything, please help.

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