My Math Forum stuck on a problem

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 October 6th, 2010, 09:45 PM #1 Member   Joined: Sep 2010 Posts: 55 Thanks: 0 stuck on a problem Suppose u is an angle in the second quadrant and sin u=3/4. So cos u=? I'm lost on this one. How do I find cos u? I know that it is negative, but I don't know how to find the answer.
 October 6th, 2010, 09:48 PM #2 Senior Member   Joined: Feb 2009 From: Adelaide, Australia Posts: 1,519 Thanks: 3 Re: stuck on a problem The equation sin²u + cos²u = 1 is true for all u.
 October 6th, 2010, 10:16 PM #3 Member   Joined: Sep 2010 Posts: 55 Thanks: 0 Re: stuck on a problem So sin^2 is 9/16. cos^2=7/16. So cos u=sqrt(7)/4. I keep putting that in but it isn't right. What am I doing wrong?
 October 6th, 2010, 10:19 PM #4 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,204 Thanks: 511 Math Focus: Calculus/ODEs Re: stuck on a problem Remember to use the negative root of cos²(u) = 7/16 since u is in quadrant II.

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