2. Connect two points AB, or connect AB;

3. extend AB to point c so that BC = AB.

4.AP intercept ab = a；;

5. Take point O as the center, make an arc with a radius of 5cm, and cross ray OP at point A;

6. Take point A and point B as the center respectively, and make an arc with a radius greater than 1/2*AB, and the two arcs intersect with point C. 。