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May 31st, 2012, 12:16 PM  #1 
Member Joined: Dec 2011 Posts: 75 Thanks: 0  trigonometry practice paper
Hello all! I always find trigonometric formulas awkward, and I'm stuck on a practice paper; any help? The acute angle (theta) is such that tan (theta) = 3 / 2 a) By considering an appropriate rightangled triangle, find the exact values of sin (theta) and cos (theta) is this simply sin (theta) = 3 and cos (theta) = 2 ? or is it sin (theta) = + or  15/4 cos (theta) = + or  13/4 or something else?? I tried to follow some formulas; don't know if I did them right cos (theta) = + or  1 / square root 1 + tan^2 theta and sin (theta) = + or  tan (theta) / square root 1 + tan^2 theta b) Use the values in part (a), and trigonometric formulas, to find the exact values of cos (2 theta) and sin (2 theta) well I don't have a clue, I'm looking at different doubleangle formulas and all sorts, completely lost! They are only given two marks each question, so I assuming they're not that complex; an explanation would be handy, thanks again! 
May 31st, 2012, 12:22 PM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs  Re: trigonometry practice paper
a) Set up a right triangle such that the side opposite ? is 3 and the side adjacent is 2. What is the hypotenuse? b) What are the doubleangle identities for sine and cosine? 
May 31st, 2012, 01:25 PM  #3 
Member Joined: Dec 2011 Posts: 75 Thanks: 0  Re: trigonometry practice paper
ok for a) hypotenuse is square root 13 so sin = 3 / square root 13 and cos = 2 / square root 13 for b) I used the identity sin (2 theta) = 2 sin(theta)cos(theta) so, sin (2 theta) = 2 sin (3 / square root 13) (cos (2 / square root 13) could I put this as 2 ( answer ) or leave it open such as 2 sin answer cos answer? then for cos (2 theta) I used = 2 cos ^2  1 cos (2 theta) = 2 (cos (2 / square root 13)^2  1 Is this right? 
May 31st, 2012, 01:34 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 20,375 Thanks: 2010 
You should simplify your answers by, for example, replacing the square of the square root of 13 with 13. What are your simplified results?

May 31st, 2012, 02:08 PM  #5 
Member Joined: Dec 2011 Posts: 75 Thanks: 0  Re: trigonometry practice paper
Hey skipjack do you mean, instead of cos (2 theta) = 2 (cos (2 / square root 13)^2)  1 change to cos (2 theta) = 2 (cos ( 4 / 13 ))  1 also could other possible simplifying involve, say sin = 3 / square root 13 change to sin = 3 (square root 13) / 13 ? surds? 
May 31st, 2012, 11:48 PM  #6 
Global Moderator Joined: Dec 2006 Posts: 20,375 Thanks: 2010 
Try to be more precise: cos ? = 2/?13, so cos 2? = 2cos²?  1 = 2(4/13)  1 = 5/13. Unless you've been taught otherwise, it isn't necessary to write 2/?13 as 2?13 / 13. Using the identity sin 2? ? 2 sin ? cos ?, what simple fraction do you now get for sin 2?? 
June 1st, 2012, 06:14 AM  #7 
Member Joined: Dec 2011 Posts: 75 Thanks: 0  Re: trigonometry practice paper
so would sin 2? ? 2 sin ? cos ? be sin 2? = 2 sin ( 3 / square root 13 ) cos ( 2 / square root 13 ) sin 2? = 2 sin cos ( 6 / square root 13 ) sin 2? = sin cos ( 12 / square root 13 ) 
June 1st, 2012, 07:35 AM  #8 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs  Re: trigonometry practice paper
Not quite. We have so we replace the trigonometric functions with their values: 
June 1st, 2012, 09:26 AM  #9 
Member Joined: Dec 2011 Posts: 75 Thanks: 0  Re: trigonometry practice paper
Ah I see, I don't know why I thought that theta should only be replaced by the values found earlier (like a variable in its own right?!), but we proved that sin (theta) and cos (theta) gave certain values already, so yeah it makes sense just to replace the trig function with the value we obtained like you said! Thanks again MarkFL and skipjack for the help and not giving up on me! 

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