# Trigonometry formula

### Basic trigonometry formula

• Perp./Hyp. = sin θ
• Base/Hyp. = cos θ
• Perp./Base = tan θ
• Base/Perp. = cot θ
• Hyp./Base = sec θ
• Hyp./Perp. = cosec θ

### Baudhayana formula

( Hyp. )2 = ( Base )2 + ( Perp.)2

### Some common trigonometric Formulas

• sin(A+B) = sin A cos B + cos A sin B
• sin(A-B) = sin A cos B – cos A sin B
• cos(A+B) = cos A cos B – sin A sin B
• cos(A-B) = cos A cos B + sin A sin B
• sin 2A = 2 sin A cos B
• cos 2A = cos2 A – sin2 A
• cos 2A = 1 – 2 sin2 A
• sin(A+B) + sin(A-B) = 2 sin A cos B
• sin(A+B) – sin(A-B) = 2 cos A sin B
• cos(A+B) + cos(A-B) = 2 cos A cos B
• cos(A+B) – cos(A-B) = -2 sin A sin B

### Trigonometric Ratios

#### Trigonometry ratio table:

Table of trigonometrical ratios of some standard angels:

Angle sin θ cos θ tan θ
00 0 1 0
300 $$\frac{1}{2}$$ $$\frac{\sqrt{3}}{2}$$ $$\frac{1}{\sqrt{3}}$$
450 $$\frac{1}{\sqrt{2}}$$ $$\frac{1}{\sqrt{2}}$$ 1
600 $$\frac{\sqrt{3}}{2}$$ $$\frac{1}{2}$$ $$\sqrt{3}$$
900 1 0 $$\infty$$
1200 $$\frac{\sqrt{3}}{2}$$ $$-\frac{1}{2}$$ $$-\sqrt{3}$$
1350 $$\frac{1}{\sqrt{2}}$$ $$-\frac{1}{\sqrt{2}}$$ -1
1500 $$\frac{1}{2}$$ $$-\frac{\sqrt{3}}{2}$$ $$-\frac{1}{\sqrt{3}}$$
1800 0 -1 0
2700 -1 0 $$-\infty$$
3600 0 1 0
Angle cot θ sec θ cosec θ
00 $$\infty$$ 1 $$\infty$$
300 $$\sqrt{3}$$ $$\frac{2}{\sqrt{3}}$$ 2
450 1 $$\sqrt{2}$$ $$\sqrt{2}$$
600 $$\frac{1}{\sqrt{3}}$$ 2 $$\frac{2}{\sqrt{3}}$$
900 0 $$\infty$$ 1
1200 $$-\frac{1}{\sqrt{3}}$$ -2 $$\frac{2}{\sqrt{3}}$$
1350 -1 $$-\sqrt{2}$$ $$\sqrt{2}$$
1500 $$-\sqrt{3}$$ $$-\frac{2}{\sqrt{3}}$$ 2
1800 $$-\infty$$ -1 $$\infty$$
2700 -1 0 $$\infty$$
3600 $$\infty$$ 1 $$\infty$$

#### Relation between Trigonometric Ratios

• sin θ cosec θ = 1
• cos θ sec θ = 1
• tan θ cot θ = 1
• tan θ = sin θ/cos θ
• cot θ = cos θ/sin θ
• sin2 θ + cos2 θ = 1
• 1 + tan2 θ = sec2 θ
• 1 + cot2 θ = cosec2 θ

#### A. Trigonometric Ratios of acute angles

• Perp./Hyp. = sin θ
• Base/Hyp. = cos θ
• Perp./Base = tan θ
• Base/Perp. = cot θ
• Hyp./Base = sec θ
• Hyp./Perp. = cosec θ

#### B. Trigonometric ratios of allied angles

1. Trigonometric ratios of (-θ) in terms of (θ)

sin(-θ) = -sinθ
cos(-θ) = cosθ
tan(-θ) = -tanθ
cot(-θ) = -cotθ
sec(-θ) = secθ
cosec(-θ) = -cosecθ

2. Trigonometric ratios of (900-θ) in terms of (θ)

sin(900-θ) = cosθ
cos(900-θ) = sinθ
tan(900-θ) = cotθ
cot(900-θ) = tanθ
sec(900-θ) = cosecθ
cosec(900-θ) = secθ

3. Trigonometric ratios of (900+θ) in terms of (θ)

sin(900+θ) = cosθ
cos(900+θ) = -sinθ
tan(900+θ) = -cotθ
cot(900+θ) = -tanθ
sec(900+θ) = -cosecθ
cosec(900+θ) = secθ

4. Trigonometric ratios of (1800-θ) in terms of (θ)

sin(1800-θ) = sinθ
cos(1890-θ) = -cosθ
tan(1800-θ) = -tanθ
cot(1800-θ) = -cotθ
sec(1800-θ) = -secθ
cosec(1800-θ) = cosecθ

5. Trigonometric ratios of (1800+θ) in terms of (θ)

sin(1800+θ) = -sinθ
cos(1800+θ) = -cosθ
tan(1800+θ) = tanθ
cot(1800+θ) = cotθ
sec(1800+θ) = -secθ
cosec(1800+θ) = -cosecθ

6. Trigonometric ratios of (900+θ) in terms of (θ)

sin(2700-θ) = -cosθ
cos(2700-θ) = -sinθ
tan(2700-θ) = cotθ
cot(2700-θ) = tanθ
sec(2700-θ) = -cosecθ
cosec(2700θ) = -secθ

7. Trigonometric ratios of (900+θ) in terms of (θ)

sin(2700+θ) = -cosθ
cos(2700+θ) = sinθ
tan(2700+θ) = -cotθ
cot(2700+θ) = -tanθ
sec(2700+θ) = cosecθ
cosec(2700+θ) = -secθ

8. Trigonometric ratios of (3600-θ) in terms of (θ)

sin(3600-θ) = -sinθ
cos(3600-θ) = cosθ
tan(3600-θ) = -tanθ
cot(3600-θ) = -cotθ
sec(3600-θ) = secθ
cosec(3600-θ) = -cosecθ

9. Trigonometric ratios of (3600-θ) in terms of (θ)

sin(3600+θ) = sinθ
cos(3600+θ) = cosθ
tan(3600+θ) = tanθ
cot(3600+θ) = cotθ
sec(3600+θ) = secθ
cosec(3600+θ) = cosecθ

10. Trigonometric ratios of (n×3600±θ) in terms of (θ)

sin(n×3600±θ) = ±sinθ
cos(n×3600±θ) = cosθ
tan(n×3600±θ) = ±tanθ
cot(n×3600±θ) = ±cotθ
sec(n×3600±θ) = secθ
cosec(n×3600±θ) = ±cosecθ

#### C. Trigonometric ratios of compound angels

1. Trigonometric ratios of sum and difference of two angles

• sin(A+B) = sinA cosB + cosA sinB
• cos(A+B) = cosA cosB – sinA sinB
• sin(A-B) = sinA cosB – cosA sinB
• cos(A-B) = cosA cosB + sinA sinB

2. Transformation of product into sums of differences

• 2 sinA cosB = sin(A+B) + sin(A-B)
• 2 cosA sinB = sin(A+B) – sin(A-B)
• 2 cosA cosB = cos(A+B) + cos(A-B)
• 2 sinA sinB = cos(A+B) – cos(A-B)

3. Transformation of sum or difference into product

Suppose A+B=C and A-B=D
or $$A = \frac{C+D}{2}$$ and $$B = \frac{C-D}{2}$$

• $$sinC+sinD = 2 sin \frac{C+D}{2} cos\frac{C-D}{2}$$
• $$sinC-sinD = 2 cos \frac{C+D}{2} sin\frac{C-D}{2}$$
• $$cosC+cosD = 2 cos \frac{C+D}{2} cos\frac{C-D}{2}$$
• $$cosC-cosD = 2 sin\frac{C+D}{2} sin\frac{D-C}{2}$$

4. Trigonometric ratios of sum of more than two angles

• sin(A+B+C) = sinA cosB cos C + cosA sinB cosC + cosA cosB sinC – sinA sinB sinC
• cos(A+B+C) = cosA cosB cosC – sinA sinB cosC – sinA cosB sinC – cosA sinB sinC

#### D. Trigonometric ratios of multiple and sub-multiple angles

Multiple angles: 2A, 3A, 4A ……

Sub-multiple angles : $$\frac{A}{2}, \frac{A}{3}, \frac{A}{4}$$…….

1. Trigonometric ratios of an angle 2A in terms of angle A

• sin2A = 2sinA cosA
• cos2A = 1-2sin2A
• $$\ tan2A = \frac{2tanA}{1-tan^2A}$$

2. Trigonometric ratios of sin2A and cos2A in terms of tanA

• $$\ sin2A = \frac{2tanA}{1+tan^2A}$$
• $$\ cos2A = \frac{1-tan^2A}{1+tan^2A}$$

3. Trigonometric ratios of an angle 3A in terms of angle A

• sin3A = 3 sinA – 4sin3A
• cos3A = 4 cos3A – 3 cosA
• $$\ tan3A = \frac{3 tanA – tan^3A}{1-3 tan^2A}$$

4. Trigonometric ratios of an angle 180

• $$sin18^0 = \frac{-1+\sqrt{5}}{4}$$
• $$cos18^0 = \frac{\sqrt{10+2\sqrt{5}}}{4}$$

5. Trigonometric ratios of an angle 360

• $$cos36^0 = \frac{1+\sqrt{5}}{4}$$
• $$sin36^0 = \frac{\sqrt{10-2\sqrt{5}}}{4}$$

6. Trigonometric ratios of an angle A in terms of angle A/2.

• $$sinA = 2 sin\frac{A}{2}cos\frac{A}{2}$$
• $$cosA = 1- 2 sin^2\frac{A}{2}$$
• $$tanA = \frac{2tan\frac{A}{2}}{1-tan^2\frac{A}{2}}$$
• $$sinA = \frac{2tan\frac{A}{2}}{1+tan^2\frac{A}{2}}$$
• $$cosA = \frac{1-tan^2\frac{A}{2}}{1+tan^2\frac{A}{2}}$$

7. Trigonometric ratios of an angle $$\frac{A}{2}$$ in terms of cosA

• $$sin\frac{A}{2} = \pm \sqrt{\frac{1-cosA}{2}}$$
• $$cos\frac{A}{2} = \pm \sqrt{\frac{1+cosA}{2}}$$
• $$tan\frac{A}{2} = \pm \sqrt{\frac{1-cosA}{1+cosA}}$$

8. Trigonometric ratios of an angle $$\frac{A}{2}$$ in terms of sinA

• $$sin\frac{A}{2} + cos\frac{A}{2} = \pm \sqrt{1+sinA}$$
• $$sin\frac{A}{2} – cos\frac{A}{2} = \pm \sqrt{1-sinA}$$

### Series Expnsion of Trigonometric functions

• sin θ = θ – θ3/3! + θ5/5! – θ7/7! …..
• cos θ = 1 – θ2/2! + θ4/4! – θ6/6! …..
• tan θ = θ + θ3/3 + 2θ5/15 …..

If is θ small

• sin θ ≈ θ
• cos θ ≈ 1
• tan θ ≈ θ

### Average Value

• < sin θ > = < sin nθ > = 0
• < cos θ > = < cos nθ > = 0
• < sin2 θ > = < sin2 nθ > = 1/2
• < cos2 θ > = < cos2 nθ > = 1/2