Trigonometry

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Hi, I need help with this question:

Use the addition and subtraction formulas ti fubd the exact values of the trigonometric ratios:

cos(-7pi/12)

Okay, I'm just having trouble with a rather basic part of the question...

I wrote cos(pi/4 + pi/3), but now I realized that the trig ratio is negative. How do I write -7pi/12 as the sum/difference of two trig ratios?

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