Why do historians refer to the period from 1781 to 1789 as the critical period?

This is the period known as Confederation, where the Articles of Confederation were drawn up - the first constitution of the United States. This was a critical period in history as there was much opposition between federalists who argued for a strong central government and anti-federalists who argued for more local powers. This resulted in the Constitutional Convention of 1987 and the Bill of Rights amendments to the constitution.

Use the formula: (n-2)×180, where n is the number of sides(18-2)×180= 28800, now this is the full exterior angle of 18-gon so you divide it by 18.28800÷18=160