Find △ABC, so that ∠ A = ∠ α, ∠ B = 2 ∠ α, AB = A.

Exercise:

1, do thunder AB

2. Let ∠ cab = ∠ α take a as the vertex.

3. the intercept AB on ray ab = a.

4.Do ∠ ABC = 2 ∠ α takes B as the vertex and crosses AC to C.

△ABC is what you want.