Simplify cos70°cos20°-sin70°sin20°

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CrossFit415
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Cos70°cos20°-sin70°sin20°

I can't find any of these angles that are equivalent to the basic angles. Thanks
 

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Do you know a formula for cos(a+b) in terms of the trig functions evaluated at a and b?
 
Cost(a+b)=cos a • cos b - sin a • sin b
 
I don't know which angles are equivalent to 70° nor 20°
 
But you know cos(20+70)=cos(90), right?