How is each interior angle of a decagon 144

Question

Today is my exam and  i want the correct answer. 

Answer 1

decagon means 10 sides

 

(n-2)*180/n

 

n=10

 

=(10-2)*180/10

 

=8*180 /10

=144

 

 

 

 

Answer 2

A diagram helps.  Unfortunately I can't draw one with this text editor.  Draw a regular decagon then draw a line segment from the center of the decagon to each of the ten vertices.  You now have ten congruent isosceles triangles.  The central angle of each triangle (at the center) is 360°/10 = 36°.  The other two base angles are equal and sum to 180° - 36° = 144°, so each angle is 144°/2 = 77°.  The interior angle consists of two base angles = 2·77° = 144°.

Answer 3

(n-2)*180/n