A) To find the external angles of the quadrilateral we will walk around the shape measuring every turn we take. We will start on the A point, rotate clock-wise and move in the direction of point "B", when we get there we will rotate clock-wise again and walk to the direction of point "C". When we do get to the point C we will notice that we rotated 180 degrees in relation to the initial position we had in point A. Moving forwars we will now rotate clockwise and go to the poind D, rotate clock-wise again when we get there, performing all the rotations needed. We will notice that we have the same orientation from the beginning, this means that we rotated 360 degrees. In other words the sum of the external angles of the quadrilateral is 360 degrees.
B) Yes, any regular polygon will have the sum of its external angles equal to 360 degrees.
C) The internal and external angles are suplementary. This means that the sum of these angles must be equal to 180 degrees, therefore:
[tex]\begin{gathered} external\text{ = 180-internal} \\ e+f+g+h=360 \\ (180-a)+(180-b)+(180-c)+(180-d)=360 \\ a+b+c+d=180+180+180+180-360 \\ a+b+c+d=4\cdot180-2\cdot180 \\ a+b+c+d=(4-2)\cdot180 \\ a+b+c+d=2\cdot180=360 \end{gathered}[/tex]
Since each external angle is the same as "180 degrees" minus the internal angle that is close to it we can represent the sum of the external angles as 360 degrees and use the mentioned relation to convert them into internal angles. If we isolate them as a sum we will find the value of the sum of the internal angles.
D) If we look at the fith line from the solution above we will notice that the sum of internal angles is represented by "(4-2)*180", the polygon had "4" sides. This means that for one that is 5 sides we should expect that it would be "(5-2)*180" and so on. So the formula is:
[tex]\text{internal = (n-2)}\cdot180[/tex]
Where "n" is the number of sides of the polygon.
