Table of Contents
07 September 2020
Read time: 6 minutes
Introduction
You must have heard of a branch of mathematics known as Trigonometry.
This blog is on one of the widely used trigonometric functions named Cosine Function.
In mathematics, the Trigonometric Functions are realvalued functions that relate an angle of a rightangled triangle to ratios of two sides of the triangle.
These functions are widely used in geometry and all sciences such as navigation, solid mechanics, celestial mechanics, geodesy, and many others.
The six trigonometric functions are the sine, the cosine, the tangent, the cosecant, the secant, and the cotangent.
Also read:
All the trigonometric functions can be calculated from a rightangled triangle.
 "Opposite" is opposite to the angle, represented by a
 "Adjacent" is adjacent (next to) to the angle, represented by b
 "Hypotenuse" is the longest side, represented by h.
What is Cosine Function?
The cosine function is one of the primary mathematical trigonometric functions.
It is complementary to the sine function (co+sine).
Cosine function was first used in ancient Egypt in the book of Ahmes (c. 2000 B.C.). Much later F. Viète (1590) evaluated some values, E. Gunter (1636) introduced the notation "Cosi" and the word "cosinus" (replacing "complementi sinus"), and I. Newton (1658, 1665) found the series expansion for cosine.
The classical definition of the cosine function for the real angle is: "the cosine of an angle in a right‐angle triangle is the ratio of the length of the adjacent side to the length of the hypotenuse."
Here, cos theta = adjacent/hypotenuse
This description of cos is valid for 0 < θ < pi/2 when the triangle is nondegenerate.
In a right triangle, we see that there are three variables: the measure of the angle, and the lengths of the two sides (Adjacent and Hypotenuse). So if we know any two of them, we can find the third variable by using the above formula.
Example 1 
PR = 26 cm and PQ = 30 cm, find theta?
Solution: From the formula, we know that the cosine of an angle is the adjacent side divided by the hypotenuse. So we can calculate Cos = 26/30
Cosine of theta is 0.866 
Example 2 
Find angle A?
Solution: In the figure above, three sides are given as AB = 3 cm, BC = 1 cm and AC = 2cm. In a right triangle, cos A = adjacent/hypotenuse = AB/AC = √3/2
cos A is √3/2 at A = 30° 
Example 3 
In the figure, find x.
Solution: Using cos formula we get cos 30° = adjacent/hypotenuse = x/15
√3/2 = x/15
Solving this we get x = (15) x (√3/2) = 7.5 x 1.73
= 12.975 units

What are the basic properties of Cosine Function?
Important cos Identities
cos^{2} (x) + sin^{2} (x) = 1
cos (x) = 1/sec (x)
cos (−x) = cos (x)
arc cos (cos (x)) = x + 2kπ [where k is an integer]
cos (2x) = cos^{2} (x) – sin^{2} (x)
cos (x) = sin (π/2 − x)
Example 4 
If cos x is 34, find sin x and hence cos 2x.
Solution: Using the identity, cos^{2} (x) + sin^{2} (x) = 1
We get, sin x = √(1cos2x) = √(19/16) = √7/4.
cos2x = os^{2} (x) – sin^{2} (x) = (3/4)^{2}(√7/4)^{2 }= (97)/16 = 1/8
cos 2x = 1/8 
All Trigonometric Functions in Terms of cosine
Any trigonometric function can be converted in terms of any other trigonometric function. Similarly, all trigonometric functions can be converted in terms of cosine function. The table below gives the formulas of five trigonometric functions in terms of cos function.
Trigonometric Functions 
Represented as cosine 
sin θ 
±√(1cos^{2}θ) 
tan θ 
±√(1cos^{2}θ)/cos θ 
cot θ 
±cos θ/√(1cos^{2}θ) 
sec θ 
±1/cos θ 
cosec θ 
±1/√(1cos^{2}θ) 
Cosine Table
Cosine Degrees 
Values 
cos 0° 
1 
cos 30° 
√3/2 
cos 45° 
1/√2 
cos 60° 
1/2 
cos 90° 
0 
cos 120° 
1/2 
cos 150° 
√3/2 
cos 180° 
1 
cos 270° 
0 
cos 360° 
1 
Cosine Properties With Respect to the Trigonometry Quadrants
The value of all trigonometric functions changes in various quadrants. In the table above, we can observe that cos 120°, 150°, and 180° have negative values while cos 0°, 30°, etc. have positive values. For cos x, which depends on the sign of x, the value will be positive in the first and the fourth quadrants and negative in the second and the third quadrants.
How is the Cosine Graph?
The trigonometric ratios can also be considered as functions of a variable which is the measure of an angle.
The angle for all trigonometric functions may vary as any real number.
Using the values of cos at various angles in different quadrants, the graph of cosine function can be plotted and it comes out a continuous function as shown below.
The cos graph comes out to be continuous, periodic, and symmetric about yaxis.
Range, Domain of Cosine and other Properties
Using a unit circle, a circle with radius 1 unit, we can calculate the domain and range of cos(x). For any point on a unit circle, the cos(x) is equal to adjacent / 1.
The measure of adjacent can be defined for all the points on the circle, indicating that the angle x can take any value, positive or negative.
So, the domain of cos(x) is all real numbers.
Also, the value of cos(x), depending on the point on the circle, can go to a maximum of 1 at x = 0֩ and a minimum of 1 at x = 180֩. So, the range of cos(x) is from 1 to 1.
The value of cos(x) changes periodically.
In short, for y = cos (x):
Domain of Cosine
Domain of Cosine = ( ∞, + ∞) = Set of all Real numbers
Range of Cosine
Range of Cosine = [1, +1]
Period of Cosine
2pi
xintercepts
x = k pi/2, place k is an integer.
yintercepts
y = 1
Symmetry
since cos (x) =  cos (x) then cos (x) is an odd function and the graph of cosx is symmetric with respect to the origin.
Summary
Cosine function also known as cos x, cos theta, or cos function is basically one of the 6 trig functions.
Cos x is the ratio of the length of the opposite to the length of the adjacent.
It has various identities that help in interconversion of the rest of the ratios into cosine.
Cos theta values change their sign in the four trigonometry quadrants, the values are positive in first and fourth quadrants while negative in the other two.
The graph of cosine comes out to be smooth, periodic and symmetric. It can be used to derive various other properties of cosine functions.
Written by Anupama Mahajan, Cuemath Teacher
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Frequently Asked Questions (FAQs)
Opposite of cos?
sec theta = 1/ cos theta
cos(x)?
cos(x) = cos (x)
cos 0
The value of cos 0 degrees is 1
cos 30?
The value of cos 30 degrees is √3/2
cos 45?
The value of cos 45 degrees is 1/√2
cos 60?
The value of cos 60 degrees is 1/2
cos 90?
The value of cos 90 degrees is 0
cos 180?
cos 180 is 1