October 6th, 2010, 10:45 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, 10:48 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, 11:16 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, 11:19 PM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 464 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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