Introduction to Trigonometry — Part 1: Ratios and standard angles: visual lesson, CBSE Class 10 Mathematics (Standard 041 / Basic 241)

Lesson

Introduction to Trigonometry, Part 1: Ratios of an angle

Trigonometry compares the sides of a right triangle, as seen from one of its acute angles.

In this lesson you will

Every ratio compares two sides, seen from the marked angle.

Concept

Define $\sin$ , $\cos$ , $\tan$ , $\operatorname{cosec}$ , $\sec$ , $\cot$ of an acute angle
Explain why they depend only on the angle
Find all six ratios from one
Recall the values at $0^\circ$ , $30^\circ$ , $45^\circ$ , $60^\circ$ , $90^\circ$
Evaluate expressions and find unknown angles

Diagram

Names of sides depend on which acute angle you stand at.

In triangle $ABC$ , right-angled at $B$ , seen from $\angle A$ : $BC$ is opposite, $AB$ is adjacent, and $AC$ is the hypotenuse.