Answer:
θ ≈ {1.6705, 4.8121}
Explanation:
You want the solutions to the equation tan(θ) = -10.
Solution
A graphing calculator or scientific calculator can tell you these values.
A scientific calculator will tell you the inverse tangent of -10 is a negative number. To put the value into the desired range, multiples of pi must be added.
tan(θ) = -10
θ = arctan(-10) ≈ -1.471128 +nπ
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Additional comment
The angle units used here are radians, since the desired interval is expressed in radians. (Not [0, 360°).)