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


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