If 0 < θ < π/2, then 0 < θ/2 < π/4, and for such angles we expect sin(θ/2) to be positive. Also, we know both sin(θ) and cos(θ) will be positive.
Given that tan(θ) = 2, we can find sec(θ) from the Pythagorean identity:
tan²(θ) + 1 = sec²(θ) ==> sec(θ) = √(tan²(θ) + 1)
… ==> cos(θ) = 1/√(tan²(θ) + 1)
… ==> cos(θ) = 1/√5
Now, recall the half-angle identity for sine:
sin²(θ/2) = (1 - cos(θ))/2
==> sin(θ/2) = √[(1 - cos(θ))/2]
==> sin(θ/2) = √[(1 - 1/√5)/2]
==> sin(θ/2) = √[(√5 - 1)/(2√5)]
==> sin(θ/2) = √[(5 - √5)/10]
