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If sin a = 3/5, find cos a and tan a

  • Last Updated : 10 Nov, 2021

Trigonometry is a discipline of mathematics that studies the relationships between the lengths of the sides and angles of a right-angled triangle. Trigonometric functions, also known as goniometric functions, angle functions, or circular functions, are functions that establish the relationship between an angle to the ratio of two of the sides of a right-angled triangle. The six main trigonometric functions are sine, cosine, tangent, cotangent, secant, or cosecant.

Angles defined by the ratios of trigonometric functions are known as trigonometry angles. Trigonometric angles represent trigonometric functions. The value of the angle can be anywhere between 0-360°.

As given in the above figure in a right-angled triangle:

  • Hypotenuse: The side opposite to the right angle is the hypotenuse, It is the longest side in a right-angled triangle and opposite to the 90° angle.
  • Base: The side on which angle C lies is known as the base.
  • Perpendicular: It is the side opposite to angle C in consideration.

Trigonometric Functions

Trigonometry has 6 basic trigonometric functions, they are sine, cosine, tangent, cosecant, secant, and cotangent. Now let’s look into the trigonometric functions. The six trigonometric functions are as follows,

  • sine: It is defined as the ratio of perpendicular and hypotenuse and It is represented as sin θ
  • cosine: It is defined as the ratio of base and hypotenuse and it is represented as cos θ
  • tangent: It is defined as the ratio of sine and cosine of an angle. Thus the definition of tangent comes out to be the ratio of perpendicular and base and is represented as tan θ
  • cosecant: It is the reciprocal of sin θ and is represented as cosec θ.
  • secant: It is the reciprocal of cos θ and is represented as sec θ.
  • cotangent: It is the reciprocal of tan θ and is represented as cot θ.

According to the above image, Trigonometric Ratios are

Sin θ = Perpendicular / Hypotenuse = AB/AC

Cosine θ = Base / Hypotenuse = BC / AC

Tangent θ = Perpendicular / Base = AB / BC

Cosecant θ = Hypotenuse / Perpendicular = AC/AB

Secant θ = Hypotenuse / Base = AC/BC

Cotangent θ = Base / Perpendicular = BC/AB

Reciprocal Identities



Sin θ = 1/ Cosec θ                    OR        Cosec θ = 1/ Sin θ

Cos θ = 1/ Sec θ                       OR        Sec θ = 1 / Cos θ

Cot θ = 1 / Tan θ                     OR         Tan θ = 1 / Cot θ

Cot θ = Cos θ / Sin θ               OR         Tan θ = Sin θ / Cos θ

Tan θ.Cot θ = 1

Values of Trigonometric Ratios

 30°45°60°90°
Sin θ 01/21/√2√3/21
Cos θ1√3/21/√21/20
Tan θ01/√31√3Not Defined
Sec θNot Defined2√22/√31
Cosec  θ12/√3√22Not Defined
Cot θNot Defined√311/√30

Trigonometric Identities of Complementary and Supplementary Angles

  • Complementary Angles: Pair of angles whose sum is equal to 90°
  • Supplementary Angles: Pair of angles whose sum is equal to 180°

Identities of Complementary angles are

sin (90° – θ) = cos θ

cos (90° – θ) = sin θ



tan (90° – θ) = cot θ

cot (90° – θ) = tan θ

sec (90° – θ) = cosec θ

cosec (90° – θ) = sec θ

Identities of supplementary angles

sin (180° – θ) = sin θ

cos (180° – θ) = – cos θ

tan (180° – θ) = – tan θ

cot  (180° – θ) = – cot θ

sec (180° – θ) = – sec θ

cosec (180° – θ) = – cosec θ

Quadrants of trigonometry

QUADRANTS

If sin a = 3/5 find cos a and tan a

Solution:

In right angled triangle 

We have sin a = 3/5

therefore Sin θ = Perpendicular / Hypotenuse = AB/AC

therefore AB = 3 

                 AC =  5

therefore as per the Pythagoras theorem 



we have AC2 = BC2 + AB2

                    52   = BC2 + 32

               25  = BC2 + 9

             25-9 = BC2

                 16 = BC2

                 BC = 4

So now we have AB = 3

                          AC = 5

                          BC = 4

Now to find cos a and tan a 

Cosine θ = Base / Hypotenuse = BC / AC

           so cos a = 4/5

and Tangent θ = Perpendicular / Base = AB / BC

          so tan a = 3/4

Similar Questions

Question 1: Prove the trigonometric equation: cos theta sec theta/cot theta = tan theta?

Solution:

Here we have cos theta sec theta / cot theta = tan theta

Therefore { cos θ sec θ }/ cot θ = tan θ

By taking L.H.S

cos θ sec θ / cot θ



we can write cos θ sec θ as 1  

       = (cos θ sec θ)/cot θ

       = 1/cot θ                                   { Cos θ = 1/ Sec θ   therefore Cos θ Sec θ  = 1}

       = tan θ                                            { Tan θ = 1 / Cot θ }

Therefore LHS = RHS

{cos θ sec θ}/ cot θ = tan θ

Hence Proved

Question 2: If tan a = 4/5 find cos a and sin a?

Solution:  

In right angled triangle

We have tan a = 3/4

Therefore Tangent θ = Perpendicular / Base = AB / BC

                AB = 3

                BC = 4

Therefore as per the Pythagoras theorem

We have     AC2 = BC2 + AB2

                   AC2 = 42 + 32

                  AC2  = 16+ 9

                  AC2 = 25

                   AC = 5                                    

So now we have AB = 3

                           AC = 5

                           BC = 4

Now to find cos a and sin a

Cosine θ = Base / Hypotenuse = BC / AC

 so cos a = 4/5

and Sin θ = Perpendicular / Hypotenuse = AB/AC

   so sin a = 3/5


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