In RT △ ABC, ∠ C = 90 ゜, with the help of your two trigonometric rulers, according to the definition of acute trigonometric function, four trigonometric function values of ∠ a are obtained （1）∠A＝30゜ （2）∠A＝60゜ （3）∠A＝45゜.

In RT △ ABC, ∠ C = 90 ゜, with the help of your two trigonometric rulers, according to the definition of acute trigonometric function, four trigonometric function values of ∠ a are obtained （1）∠A＝30゜ （2）∠A＝60゜ （3）∠A＝45゜.

If you already know, ∠ a = 30 ゜, ∠ a = 60 ゜, ∠ a = 45 ゜, you should know that when ∠ a = 30 ゜, sine = 1 / 2, cosine = root 3 / 2, tangent = root 3 / 3, Cotangent = root 3 ゜, sine = root 3 / 2, cosine = 1 / 2, tangent = root 3, Cotangent

Acute triangle function
If the angle a is an internal angle of an equilateral triangle, then the value of cosa is equal to ()
A 1 / 2 B root 2 / 2 C and 3 / 2 D 1

a 1/2

Why cos (- 60 °) = cos60 ° why can't it be equal to - cos60

Because y = cosx, this function is even
It's about Y-axis symmetry
So cos (- x) = cosx
COS（-60°）=COS60°