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**Definitions and Formulas**

- An angle is positive if it is measured in the anticlockwise direction and negative if it is measured in the clockwise direction.
- In a right triangle ABC, right-angled at B

**Reciprocal Relations**

- If one of the trigonometric ratios of an acute angle is known, the remaining trigonometric ratios can be determined.
- Two angles are said to be complementary if their sum is 90°, and each one of them is called the
**complement of the other**.

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

An equation with trigonometric ratios of an angle θ, which is true for all values of ‘θ’, for which the given trigonometric ratios are defined, is called an **identity**.**Three fundamental trigonometric identities are****(a)** sin^{2}θ + cos^{2}θ = 1**(b)** sec^{2}θ = 1 + tan^{2}θ**(c) **cosec^{2}θ = 1 + cot^{2}θ

**Table**: Values of trigonometric ratios of 0°, 30°, 45°, 60° and 90°

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