# What is the value of cos 180°?

Cosine is a primary trigonometric function that represents the ratio between the lengths of base and hypotenuse in a right triangle.

## Answer: The value of cos 180° is -1.

Let's find cos 180°.

**Explanation:**

We know that cos 180° = cos π

⇒ cos (π - θ ) = -cos θ

⇒ cos (π - 0º ) = -cos 0º

⇒ cos 0º = 1, but cos 180° lies in the second quadrant.

The value of the function cosine is positive in the first and fourth quadrant only.

Therefore, cos 180° is -1.