Unit Circle

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.

Unit Circle

QOP and T'OS' are similar triangles.


Trigonometric Ratios

Trigonometric Ratios

Trigonometric Ratios

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:

Unit Circle Ratios

-1 ≤ sen α ≤ 1

-1 ≤ cos α ≤ 1