`cos(130)cos(40) - sin(130)sin(40)` Write the expression as a sine, cosine, or tangent of an angle.

You need to recognize the formula `cos(a+b) = cos a*cos b - sin a*sin b.` You need to put a = `130^o` and b = `40^o` , such that:

`cos 130^o*cos 40^o - sin 130^o sin 40^o = cos (130^o+40^o)`

`cos (130^o +40^o) = cos 170^o `

Hence, the given expression could be evaluated as the cosine of the sum of angles `130^o` and` 40^o,` such that `cos (130^o + 40^o) = cos 170^o ` .

You need to recognize the formula `cos(a+b) = cos a*cos b - sin a*sin b.` You need to put a = `130^o` and b = `40^o` , such that:

`cos 130^o*cos 40^o - sin 130^o sin 40^o = cos (130^o+40^o)`

`cos (130^o +40^o) = cos 170^o `

Hence, the given expression could be evaluated as the cosine of the sum of angles `130^o` and` 40^o,` such that `cos (130^o + 40^o) = cos 170^o ` .