SOLUTION: Please help me solve this equation for u: cos2u-7cosu+7=0 with [0,2pi) as a restriction.

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Question 775868: Please help me solve this equation for u: cos2u-7cosu+7=0 with [0,2pi) as a restriction.
Answer by lwsshak3(11628) About Me  (Show Source):
You can put this solution on YOUR website!
solve this equation for u: cos2u-7cosu+7=0 with [0,2pi) as a restriction.
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cos%282u%29-7cosu-7=0
cos%5E2%28u%29-sin%5E2%28u%29-7cosu-7=0
cos%5E2%28u%29-1%2Bcos%5E2%28u%29-7cosu-7=0
2cos%5E2%28u%29-7cosu-8=0
solve for cos(u) by quadratic formula:
cos%28u%29+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
a=2, b=-7, c=-8
cos(u)≈4.41(reject, (-1 < cos(u) < 1)
or
cos(u)≈-0.9075
u=0.4335, 3.575 (in Q3 and Q4 where cos<0)