
Trigonometry Trigonometry Math Forum 
 LinkBack  Thread Tools  Display Modes 
December 3rd, 2016, 07:01 AM  #1 
Newbie Joined: Dec 2016 From: Ireland Posts: 4 Thanks: 0  Trig Identities (please help!)
In the solution to a question, it says that 2sinAcosA.sinA is equal to 2cosA(1  cos^2A). How are they equal? I can't figure it out! (Note that the question I am doing is to prove that cos3A = 4cos^3A  3cosA.) Last edited by skipjack; December 3rd, 2016 at 08:24 AM. 
December 3rd, 2016, 07:20 AM  #2 
Math Team Joined: Dec 2013 From: Colombia Posts: 6,854 Thanks: 2228 Math Focus: Mainly analysis and algebra 
$$2\sin{A}\, \cos{A}\, \sin{A} = 2 \cos{A}\, \sin{A}\, \sin{A} = 2 \cos{A}\, \sin^2{A} = 2\cos{A}\,(1\cos^2{A})$$
Last edited by v8archie; December 3rd, 2016 at 07:23 AM. 

Tags 
homework, identities, maths, trig, trigonometry 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
trig identities  sara213  Trigonometry  14  August 5th, 2011 06:05 AM 
Trig Identities  Gokias  Trigonometry  10  July 13th, 2011 02:14 PM 
Trig Identities  jelsier  Algebra  2  May 25th, 2009 07:56 PM 
Trig Identities  kevv2692  Algebra  2  May 25th, 2009 07:29 PM 
More trig identities  jordanshaw  Calculus  5  January 1st, 1970 12:00 AM 