Question 1197838
Find all solutions in the interval [0,2pi) 
Cos^2 theta -9 cos theta - 1 =0
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If you mean Cos^2(t) -9cos(t) - 1 = 0
Sub x for cos(t)
{{{x^2 - 9x - 1 = 0}}}
*[invoke solve_quadratic_equation 1,-9,-1]
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Ignore the value >1, no real solution.
cos(t) =~ -0.109772228
theta = ~96.3022 degs
theta = ~263.6978 dega
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Convert to radians if you like.