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 March 11th, 2010, 10:59 PM #1 Newbie   Joined: Mar 2010 Posts: 1 Thanks: 0 Brain fried from trig angles question Hi everyone, I am currently stuck on this extension question from my textbook: If sin(x) = 1/4 where sin(x) is 0 < x < pi/2 and sec(y) = 6/5 with 0 < y < pi/2, find the exact value of sin(x + y). What I have done is: Use Trig identity: sin(x+y) = sin(x)cos(y) + sin(y)cos(x) sec(y) = 6/5 => 1/cos(y) = 6/5 => cos(y) = 5/6 Therefore... sin(x+y) = (1/4 * 5/6) + cos(x)sin(y) And I kind of get stuck since I'm not sure how to find cos(x) or sin(y). I'm thinking of using the identities, sin^2(x) + cos^2(x) = 1 and sin^2(y) + cos^2(y) = 1, to find what cos(x) and sin(y) are respectively but I am not confident that I am going in the right direction. Thank you in advance. March 12th, 2010, 06:04 AM #2 Senior Member   Joined: Mar 2007 Posts: 428 Thanks: 0 Re: Brain fried from trig angles question If I had a right triangle and knew that the sine of an angle in it was 1/4, would that not give me an idea of the possible [proportionate] lengths of two sides? Would that not then enable me to figure the 3rd side using Pythagoras, then all the other ratios? March 13th, 2010, 08:22 AM #3 Member   Joined: Dec 2009 Posts: 66 Thanks: 0 Re: Brain fried from trig angles question Like David stated, draw a right triangle out and fill in the known proportions. From there it will be a lot more easier to understand your dilemma March 22nd, 2010, 08:15 AM #4 Newbie   Joined: Mar 2010 Posts: 10 Thanks: 0 Re: Brain fried from trig angles question I think you're heading in the right direction. cos(x) = +-squareroot( 1 - sin^2(x) ) = +-squareroot( 15/16 ) sin(y) = +-squareroot( 1 - cos^2(y) ) = +-squareroot( 11/36 ) Tags angles, brain, fried, question, trig Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Threads Thread Thread Starter Forum Replies Last Post MCity98 Trigonometry 1 September 16th, 2013 05:51 PM wyliec Trigonometry 4 September 1st, 2012 06:35 AM dunn Trigonometry 4 October 2nd, 2011 10:37 AM unforgiving Algebra 2 February 4th, 2008 11:27 AM rune2402 Applied Math 7 December 16th, 2007 08:54 AM

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