The unit circle has its center at (0, 0) and its radius is one.
In the unit circle, the coordinate axes delimit four quadrants that are numbered in an anti-clockwise direction.
By drawing a triangle on the unit circle, straight lines called line values can be drawn whose length represents the value of a trigonometric function.
QOP and T'OS' are similar triangles.
The sine is the ordinate, that is to say, the y-coordinate or the vertical value in a pair of coordinates.
The cosine is the abscissa, that is to say, the x-coordinate or the horizontal value in a pair of coordinates.
From this data, the sign of the trigonometric ratios in each quadrant are as follows:
-1 ≤ sen α ≤ 1
-1 ≤ cos α ≤ 1