trig equation help

[CrossFit415]

Cos70°cos20°-sin70°sin20°

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

[Dick]

Do you know a formula for cos(a+b) in terms of the trig functions evaluated at a and b?

[CrossFit415]

Cost(a+b)=cos a • cos b - sin a • sin b

[CrossFit415]

I don't know which angles are equivalent to 70° nor 20°

[Dick]

But you know cos(20+70)=cos(90), right?

[CrossFit415]

Whoops. Thanks