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 September 13th, 2009, 04:24 PM #1 Member   Joined: Feb 2008 Posts: 30 Thanks: 0 Solve X where -infinity>x>infinity Sinx + 1 = 2Cos^2x I got pi/6 + 2piK 5pi / 6 + 2piK -pi/2 + 2pik but it says -pi/2 + 2pik isnt an answer? WHY? September 13th, 2009, 05:05 PM   #2
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Re: Solve X where -infinity>x>infinity

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 Originally Posted by -DQ- Sinx + 1 = 2Cos^2x I got pi/6 + 2piK 5pi / 6 + 2piK -pi/2 + 2pik but it says -pi/2 + 2pik isnt an answer? WHY?
The equation as written doesn't seem right. Your first 2 solutions do not satisfy the equation, while the third one does. September 13th, 2009, 05:15 PM #3 Member   Joined: Feb 2008 Posts: 30 Thanks: 0 Re: Solve X where -infinity>x>infinity Using trig identities i made it Sinx + 1 = 2 - 2Sin^2x Sin^2x + Sinx - 1 = 0 (2Sinx-1)(Sinx+1)=0 Sinx= 1/2, -1 September 13th, 2009, 06:54 PM #4 Global Moderator   Joined: Dec 2006 Posts: 20,966 Thanks: 2216 Your second line above should have been 2Sin^2x + Sinx - 1 = 0, but everything else you did is correct. Assuming the problem was given correctly, your textbook (or whatever) seems to be incorrect; also, mathman made a slip. September 13th, 2009, 07:14 PM #5 Member   Joined: Feb 2008 Posts: 30 Thanks: 0 Re: Solve X where -infinity>x>infinity Yes, it is 2 sin^2x.. typo. For some reason I have a feeling theres a reason why -pi/2 + 2piK wasn't included. September 14th, 2009, 05:13 AM #6 Senior Member   Joined: May 2008 From: York, UK Posts: 1,300 Thanks: 0 If the original problem were then would not be a solution (I hope you see why). Is this relevant, or not? Tags infinity>x>infinity, solve 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 Real_nimr0d Calculus 5 March 4th, 2014 02:19 PM fibonaccilover Applied Math 9 July 26th, 2009 08:07 PM conjecture Calculus 1 April 18th, 2008 03:31 PM sixflags11804 Calculus 4 April 1st, 2008 05:24 PM nerd9 Number Theory 1 December 31st, 1969 04:00 PM

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