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# COS 120 DEGREES

Trigonometry is the study of relationships between sides and angles of a triangle. It is used widely in different fields like astronomy, geography, navigation, engineering, etc.

The values of sine, cosine, etc., are mostly used in all the above-discussed fields. We will be discussing more cosine and mainly about how to find the value of cos 120o.

Let us consider a right triangle ABC. Here ∠ ACB is an acute angle. From the above right triangle, AC is the hypotenuse, BC is the adjacent side, and AB is the opposite side to angle C. Let us now see the values of trigonometric ratios of cos 0°, 30°, 45°, 60°,90°.

 cos 0° = 1 cos 30° = √3/2 cos 45° = 1/√2 cos 60° = 1/2 cos 90° = 0

Now, let us find out the value of cos 120°.

## VALUE OF COS 120°

Let us discuss a small concept of about two important and useful rules, which makes it easy to find the value of cos 120 degrees.

It says that the angles which lie between 0°− 90° are said to be in the first quadrant, angles between 90°− 180° be in the second quadrant, angles between 180°− 270° be in the third quadrant, and angles between 270°− 360° be in the fourth quadrant. This is called the quadrant rule.

## CAST RULE:

This rule mainly helps us to memorize the quadrants at which sinθ, cosθ, and tanθ are positive.

Second quadrant: Only the function sine is positive. Quadrant denoted As S.

If we carefully observe here, starting from 4th quadrant to 1st quadrant, it is in the path of CAST, where,

C = Cosine.        A = All.

S = Sine.        T = Tangent.

From the above figure, 120° lies between 90o and 180°.Since 120°> 90° and 120°< 180°, so we can express 120° as (90° + 30°) and (180° − 60°).

1. (90° + 30°)

Cos 120° = Cos (90° + 30°)

We know, Cos (90° + θ) = − Sin θ

Cos (90° + 30°) = − Sin 30°

We also know, Sin 30° = ⇒− Sin 30° =−1/2

Cos 120° = −1/2

2. (180° − 60°)

Cos 120° = Cos (180° − 60°)

We know, Cos (180° −θ) = − cos θ

Cos (180° − 60°) = − cos 60°

We also know, cos 60° = ⇒− cos 60° = −1/2

Cos 120° = −1/2

Hence, the value of Cos 120° =−1/2

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