Construct an isosceles triangle given a side of length r and a vertex angle with measure α. 
Use the sliders to determine the  length r and measure α. 
(The congruent sides have length r)
You can use the tool that constructs angles of a given length.
Write down the steps of your construction in the spreadsheet, and a 
proof that you obtained the desired triangle.

r = 5.2 (slider from 0 to 10, step 0.1)
α = 24.3 (slider from 0 to 180, step 0.1)
b_1: abs(r)

Tool: Angle with given size (command Anglewithgivensize)
Input: B, A, α
Output: γ, c

B' = Rotate(B, α°, A)
c = Ray(A, B')
γ = Angle(B, A, B')