Express sin(3theta) in terms of sin (theta)

First of all:-

3∅ can be called (2∅+∅)
so, sin(3∅) = sin(2∅ + ∅)
Open brackets on the RHS (right hand side)
 = sin2∅ cos ∅ + cos2∅sin∅
This is simplified into;
 = 2sin∅cos^2∅ + sin∅ (1- 2sin^2∅)
 = sin∅ (2cos^2∅ + 1 - 2sin^2∅)
 = sin∅ (2(1 - sin^2∅) +1-2sin^2∅)
 = 3sin∅ - 4sin^3∅