# If A =30o, then prove that : sin 3A = 3 sin A - 4 sin3A - Mathematics

Sum

If A =30o, then prove that :
sin 3A = 3 sin A - 4 sin3A.

#### Solution

Given A = 30°

sin 3A = sin 3(30°)
= sin 90°
=1

3 sin A – 4 sin3A = 3 sin 30° – 4 sin330°

=3(1/2) – 4(1/2)^3

= (3)/(2) – (1)/(2)

= 1

∴ sin 3A = 3 sin A – 4 sin3A

Concept: Trigonometric Ratios of Some Special Angles
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#### APPEARS IN

Selina Concise Mathematics Class 9 ICSE
Chapter 23 Trigonometrical Ratios of Standard Angles [Including Evaluation of an Expression Involving Trigonometric Ratios]
Exercise 23 (B) | Q 2.4 | Page 293