# TRIGONOMETRIC RATIOS OF 180 DEGREE MINUS THETA

## About "Trigonometric ratios of 180 degree minus theta"

Trigonometric ratios of 180 degree minus theta is one of the branches of ASTC formula in trigonometry.

Trigonometric-ratios of 180 degree minus theta are given below.

sin (180° - θ)  =  sin θ

cos (180° - θ)  =  - cos θ

tan (180° - θ)  =  - tan θ

csc (180° - θ)  =  csc θ

sec (180° - θ)  =  - sec θ

cot (180° - θ)  =  - cot θ

Let us see, how the trigonometric ratios of 180 degree minus theta are determined.

To know that, first we have to understand ASTC formula.

The ASTC formula can be remembered easily using the following phrases.

"All Sliver Tea Cups"

or

"All Students Take Calculus"

ASTC formla has been explained clearly in the figure given below.

More clearly

From the above picture, it is very clear that

(180° - θ) falls in the second quadrant and

In the third quadrant (180° + θ), tan and cot are positive and other trigonometric ratios are negative.

## Important conversions

When we have the angles 90° and 270° in the trigonometric ratios in the form of

(90° + θ)

(90° - θ)

(270° + θ)

(270° - θ)

We have to do the following conversions,

sin θ <------> cos θ

tan θ <------> cot θ

csc θ <------> sec θ

For example,

sin (270° + θ)  =  - cos θ

cos (90° - θ)  =  sin θ

For the angles 0° or 360° and  180°, we should not make the above conversions.

## Evaluation of trigonometric ratios using ASTC formula

Problem 1 :

Evaluate : sin (180° - θ)

Solution :

To evaluate sin (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "sin" will not be changed as "cos"

(iii)  In the II nd quadrant, the sign of "sin" is positive.

Considering the above points, we have

sin (180° - θ)  =  sin θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

Problem 2 :

Evaluate : cos (180° - θ)

Solution :

To evaluate cos (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "cos" will not be changed as "sin"

(iii)  In the II nd quadrant, the sign of "cos" is negative.

Considering the above points, we have

cos (180° - θ)  =  - cos θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

Problem 3 :

Evaluate : tan (180° - θ)

Solution :

To evaluate tan (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "tan" will not be changed as "cot"

(iii)  In the II nd quadrant, the sign of "tan" is negative.

Considering the above points, we have

tan (180° - θ)  =  - tan θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

Problem 4 :

Evaluate : csc (180° - θ)

Solution :

To evaluate csc (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "csc" will not be changed as "sec"

(iii)  In the II nd quadrant, the sign of "csc" is positive.

Considering the above points, we have

csc (180° - θ)  =  csc θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

Problem 5 :

Evaluate : sec (180° - θ)

Solution :

To evaluate sec (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "sec" will not be changed as "csc"

(iii)  In the II nd quadrant, the sign of "sec" is negative.

Considering the above points, we have

sec (180° - θ)  =  - sec θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

Problem 6 :

Evaluate : cot (180° - θ)

Solution :

To evaluate cot (180° - θ), we have to consider the following important points.

(i)  (180° - θ) will fall in the II nd quadrant.

(ii)  When we have 180°, "cot" will not be changed as "tan"

(iii)  In the II nd quadrant, the sign of "cot" is negative.

Considering the above points, we have

cot (180° - θ)  =  - cot θ

Let us look at the next stuff on "Trigonometric ratios of 180 degree minus theta"

## Summary (180 degree minus theta)

sin (180° - θ)  =  sin θ

cos (180° - θ)  =  - cos θ

tan (180° - θ)  =  - tan θ

csc (180° - θ)  =  csc θ

sec (180° - θ)  =  - sec θ

cot (180° - θ)  =  - cot θ

After having gone through the stuff given above, we hope that the students would have understood "Trigonometric ratios of 180 degree minus theta"

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