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October 6th, 2010, 09:45 PM   #1
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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.
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October 6th, 2010, 09:48 PM   #2
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Re: stuck on a problem

The equation sin²u + cos²u = 1 is true for all u.
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October 6th, 2010, 10:16 PM   #3
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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?
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October 6th, 2010, 10:19 PM   #4
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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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