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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