cos(x), cosine function.
In a right triangle ABC the sine of α, sin(α) is defined as the ratio betwween the side adjacent to angle α and the side opposite to the right angle (hypotenuse):
cos α = b / c
b = 5″
c = 10″
cos α = b / c = 5 / 10 = 0.5
Graph of cosine
Inverse cosine function
The arccosine of x is defined as the inverse cosine function of x when -1≤x≤1.
When the cosine of y is equal to x:
cos y = x
Then the arccosine of x is equal to the inverse cosine function of x, which is equal to y:
arccos x = cos-1 x = y
arccos 1 = cos-1 1 = 0 rad = 0°