1, straight line, fixed point b, BC=a, fixed point C.

2. Draw an arc with the same radius at the vertex, point B and point C of angle A. ..

3. Take an arc with an angle of A at the intersection point D and the intersection point B and BC. ..

4. Draw a double arc at the angle A, and take a double arc at the intersection of point C and BC, the intersection point E. ..

5. the intersection of 5.BD and CE (extension line) is point a.