# Introduction to Trigonometric Functions

The trigonometric ratios we have studied till now are the ratios of acute angles in any right-angled triangle. What about trigonometric ratios for any angled triangle. When we talk about a normal triangle with any degree of angle, we extend our term to trigonometric functions. The following chapter shall extend your knowledge of Trigonometric functions for a triangle with any degree of angle.

## Introduction to Trigonometric Functions

The angle measured in a triangle for trigonometric functions is taken in terms of Radian measures. For understanding the concept of radian measure and trigonometric functions, we need to first analyse the figure given below:

The figure above is of a circle with centre O, that lies at the origin of coordinate axes. Q(a,b) is any point on the circle with angle SOQ = x radian. The length of arc SQ here fromÂ âˆ SOQ is x. Now we define cos x=a and sin x = b.

### QuadrantalÂ Angles

In the figure, we can see that ONQ is a right-angled triangle, therefore,

ON2 + NQ2 = OQ2
or a2 +b2Â = 1
or, cos2x +sin2x = 1

This signifies every point on the unit circle isÂ cos2x +sin2x. Now, we already know that one complete revolution subtends the angle of 2Ï€ radian at the centre of the circle, soÂ  Â  Â  âˆ  SOT =Â Ï€/2, similarly,Â âˆ  SOU =Â Ï€ andÂ âˆ  SOV =Â  3Ï€/2.

The angles discussed above are called quadrantal angles because these are integral multiples ofÂ Ï€/2.Â  The coordinate value of the points S,T,U and V are respectively, (1,0), (0,1), (-1,0) and (0,-1). The value of the quadrantal angles in the above circle shall be:

When we take one complete revolution from point Q, we come back to the same starting point Q.Â  Also, the increase or decrease in the value of x by an integral multiple of 2Ï€ does not affect the values of sine and cosine functions.

sin (2nÏ€ + x) = sin x, n âˆˆ Z , cos (2nÏ€ + x) = cos x , nâˆˆ Z

From this we know that, sin x = 0,Â  only ifÂ x = 0,Â Â±Ï€,Â Â±2Ï€,Â Â±3Ï€, …….. This implies that sineÂ x shall be 0 ifÂ xÂ is an integral multiple ofÂ Ï€. Further, cosÂ xÂ = 0, ifÂ x =Â Â±Ï€/2,Â Â±3Ï€/2,Â Â±5Ï€/2. This means that the value of cosÂ x becomes 0 or the value of cosÂ x vanishes whenÂ xÂ happens to be the odd multiple of Ï€/2.Â From the above discussion we can hence conclude thatÂ sin x = 0 when x = nÏ€, and cos xÂ = o when xÂ = (2n+1)Ï€/2,Â  where n is an integer.

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## Trigonometric Functions in terms of Sine and Cosine Functions

Let’s use sine and cosine functions to determine the other trigonometric functions.

• cosecÂ Â x = 1/sin x, whereÂ xÂ â‰  nÏ€
• secÂ x = 1/cosÂ xÂ  whereÂ xÂ â‰  (2n +1)Ï€/ 2
• tanÂ xÂ = sinÂ x /cosÂ x ,Â whereÂ xÂ â‰  (2n +1)Ï€/ 2
• cotÂ xÂ = cosÂ x/ sinÂ x, whereÂ xÂ â‰  nÏ€

In all the above functions, n is an integer. For all the real values ofÂ x, we already know that,

• sin2 x + cos2 x = 1,

This lets us know that,

• 1 + tan2 x = sec2 x
1 + cot2 x = cosec2 x

We already know the values of trigonometric ratios for the angles of 0Â°, 30Â°, 45Â°,60Â° and 90Â°. We use the same values for trigonometric functions as well. The values of trigonometric functions thus are as shown in the table below:

For knowing the values of cosec x, sec x and cot x we reciprocate the values ofÂ sin x, cos x and tan x, respectively. From the above discussion, we can now calculate the values of the various trigonometric functions by using the respective trigonometric ratios, as stated in the table above.

## Solved Examples For You

Question 1:Â If $$\displaystyle \frac{\pi}{2}<\alpha<\pi$$, then
$$\sqrt{\frac{1-\cos\alpha}{1+\cos\alpha}}+\sqrt{\frac{1+\cos\alpha}{1-\cos\alpha}}=?$$

Answer :

$$\sqrt{\frac{1-\cos \alpha}{1+\cos \alpha}} + \sqrt{\frac{1+\cos \alpha}{1-\cos \alpha}}$$ $$= \sqrt{\frac{(1-\cos \alpha)^{2}}{\sin ^{2}\alpha}} + \sqrt{\frac{(1+\cos \alpha)^{2}}{\sin ^{2}\alpha}}$$
$$= |\mathrm{cosec}\alpha – cot\alpha| + |\mathrm{cosec}\alpha + \cot\alpha|$$
$$= 2\mathrm{cosec}\alpha$$ because $$\sin \alpha > 0,\alpha \in(\frac{\pi}{2},\pi)$$

Question 2: What are the six basic trigonometric functions?

Answer: There are six main trigonometric functions which are Sine (sin), Cosine (cos), Tangent (tan), Secant (sec), Cosecant (csc), Cotangent (cot).

Question 3: Who invented trigonometry?

Answer: Hipparchus invented trigonometry. He was an astronomer that lived from 190-120 B.C. He is said to be the father of trigonometry that is a branch of mathematics. This branch studies the angles of sides of triangles.

Question 4: Why was trigonometry invented?

Answer: Trigonometry was invented mainly for astronomy, to be exact, for spherical geometry. The first ones to give coordinates for stars were the Babylonians. They made use of the ecliptic as their base circle in the celestial sphere, that is to say, the crystal sphere of stars.

Question 5: What are the branches of trigonometry?

Answer: The two major branches of trigonometry are plane trigonometry and spherical geometry. Further, in general, trigonometry is known to deal with the study of the relationships concerning the lengths of angles and triangles.

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