Hello! everyone

Show that sin 3A = 3 sin A – 4 sin3 A

Solution

Sin 3A= sin(2A + A)

= sin 2A cos A + cos 2A sin A

= (2sin A cos A) cos A + (1 – 2 sin2 A) sin A

= 2sin A cos2 A + sin A – 2sin3 A

= 2sin A(1 – sin2 A) + sin A – 2sin3 A

= 3sin A – 4sin3 A

Someone please let me know why + cos 2A sin A become + (1 – 2 sin2 A) sin A