Is cos 110 ° equal to 0? Is cos 90 ° equal to 0?

Is cos 110 ° equal to 0? Is cos 90 ° equal to 0?

Cos110 ° is equal to - cos70 ° and neither is equal to 0
In the range of 0 to 360, cos90 ° and cos270 ° are equal to 0

Cos α = - sin20 ° use the formula 5 cos α = sin (- 20) = cos (90 - (- 20) = cos (110) so that α equals 110 °
Why is it different to use cos = sin (- 20 °) = cos (- 20 + 90) = cos (70)? I've got something wrong and asked for the guidance of the great God

Sin (- 20 °) = - cos (- 20 + 90) you are missing a minus sign

"α = π 6 + 2K π (K ∈ z)" is the expression of "Cos2 α = 12"______ Conditions

When "α = π 6 + 2K π (K ∈ z)", "Cos2 α = 12" is tenable, when "Cos2 α = 12", "α = π 6 + 2K π (K ∈ z)" is not tenable, such as α = - π 6; therefore, "α = π 6 + 2K π (K ∈ z)" is sufficient and unnecessary of "Cos2 α = 12"; so the answer: sufficient and unnecessary