Trigonometric Identities  TOPIC_SOLVED

Trigonometric ratios and functions, the unit circle, inverse trig functions, identities, trig graphs, etc.

Trigonometric Identities

Postby jazz98 on Wed Mar 24, 2010 1:55 pm

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
jazz98
 
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Re: Trigonometric Identities  TOPIC_SOLVED

Postby midas on Wed Mar 24, 2010 5:21 pm

it's one of the double-angle identities

http://www.purplemath.com/modules/idents.htm#double

cos(2x)=1-2sin^2(x)
midas
 
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