SOLUTION: 5cos^2 theta +cos theta-1=0 Find all possible values of theta in between 0° and 360°

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Question 839010: 5cos^2 theta +cos theta-1=0 Find all possible values of theta in between 0° and 360°
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Use a substitution and the quadratic formula,
u=cos%28theta%29
.
.
.
5u%5E2%2Bu-1=0
u+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
u+=+%28-1+%2B-+sqrt%28+1%5E2-4%2A5%2A%28-1%29+%29%29%2F%282%2A5%29+
u+=+%28-1+%2B-+sqrt%28+1%2B20+%29%29%2F%2810%29+
u+=+%28-1+%2B-+sqrt%28+21+%29%29%2F%2810%29+
First solution:
u+=+%28-1+%2B+sqrt%28+1%2B20+%29%29%2F%2810%29=+0.3583
cos%28theta%29=0.3583
theta=69.0
Second solution:
u+=+%28-1+-+sqrt%28+1%2B20+%29%29%2F%2810%29=+-0.5583
cos%28theta%29=-0.5583
theta=123.9