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If A, B and C are interior angles of a triangle ABC, then show that sin [(B + C)/2] = cos A/2.

  • Last Updated : 26 Oct, 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 Defined 2√22/√31
Cosec θ  12/√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

Quadrsants

If A, B, and C are interior angles of a triangle ABC, then show that sin [(B + C)/2] = cos A/2?

Solution: 

Here We will use the trigonometric ratios of complementary angles

sin (90° – θ) = cosθ

As We know that for ΔABC,

∠A + ∠B + ∠C = 180° (Angle sum property of triangle)

∠B + ∠C = 180° – ∠A

On dividing both sides by 2, we get,

(∠B + ∠C)/2 = (180° – ∠A)/2

(∠B + ∠C)/2 = 90° – ∠A/2

Now ,Applying sin angles on both the sides:

Therefrore 

sin {(∠B + ∠C)/2} = sin (90° – ∠A/2)         { Since, sin (90° – θ) = cos θ,}

sin (∠B + ∠C)/2 = cos A/2

Hence Proved

Similar Questions

Question 1: Simplify 2 tan2 45° + cos2 30° – sin2 60°.

Solution: 

As we know,with tigonomteric value table

tan 45° = 1

cos 30° = √3/2

sin 60° = √3/2

Therefore, put these values in the given equation:

= 2(1)2 + (√3/2)2 – (√3/2)2

= 2 + 0

= 2

Question 2: If tan 2A = cot (A – 18°), where 2A is an acute angle, find the value of A.

Solution: 

Here we have

tan 2A = cot (A – 18°)

As we know by trigonometric identities,

tan 2A = cot (90° – 2A)                   {cot (90° – θ) = tan θ}

Substituting the above equation in the given equation, we get;

⇒ cot (90° – 2A) = cot (A – 18°)

⇒ 90° – 2A = A – 18°

⇒ 108° = 3A

A = 108° / 3

So, the value of A = 36°

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