let n be the number of sides,
x(1),x(2),...,x(i),x(n) be the interior angles(內角).
y(1),y(2),...,y(i),y(n) be the exterior angles(外角).
We have (n+2)equations:
x(1)+x(2)+...+x(n)=(n-2)*180...........(1) (內角總和)
y(i)=2*x(i).................(2) (內角和外角之比),i=1,2,...,n
x(1)+y(1)=360..................(3) (在一點上,內角加外角等於360度)
x(2)+y(2)=360..................(4) (同上)
:
:
x(n)+y(n)=360..................(n+2) (同上)
from equations,(3),(4),...,(n+2), we get
x(1)+x(2)+...+x(n)+y(1)+y(2)+...+y(n)=360*n
sub (2) into above,
x(1)+x(2)+...+x(n)+{2*[x(1)+x(2)+...+x(n)]}=360*n
3*[x(1)+x(2)+...+x(n)]=360*n
sub(1) into above,
3*(n-2)*180=360*n
540*(n-2)=360*n
n=6 ANS.