Answer:
0.2036
Explanation:
u = arcsin(0.391) ≈ 23.016737°
tan(u/2) = tan(11.508368°)
tan(u/2) ≈ 0.2036
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You can also use the trig identity ...
tan(α/2) = sin(α)/(1+cos(α))
and you can find cos(u) as cos(arcsin(0.391)) ≈ 0.920391
or using the trig identity ...
cos(α) = √(1 -sin²(α)) = √(1 -.152881) = √.847119
Then ...
tan(u/2) = 0.391/(1 +√0.847119)
tan(u/2) ≈ 0.2036
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Comment on the solution
These problems are probably intended to have you think about and use the trig half-angle and double-angle formulas. Since you need a calculator anyway for the roots and the division, it makes a certain amount of sense to use it for inverse trig functions. Finding the angle and the appropriate function of it is a lot easier than messing with trig identities, IMO.