<PRE><B>Question 147340</B>
Yes, you can form a corollary from the rule (n-2)180.

Case n=4 then 2*180=360.

However, a more exhaustive proof is better.

Conjecture (Quadrilateral Sum ): The sum of the measures of the interior angles in any convex quadrilateral is 360 degrees.

Proof: We can break a convex quadrilateral into 2 triangles. Because the angles in a triangle add to 180 degrees by a Triangle Sum Conjecture, we have 2*180=360.

QED</PRE>