Polygons

   10 INK 7: PAPER 0: BORDER 0: CLS 
   20 PRINT "diagonals of a polygon"
   30 LET i=6.3
   40 DIM x(25): DIM y(25)
   50 FOR n=4 TO 20
   60 LET x(1)=128: LET y(1)=90
   70 FOR a=1 TO n
   80 LET x(a+1)=128+70*SIN (i*a/n)
   90 LET y(a+1)=90+70*COS (i*a/n)
  100 NEXT a
  110 CLS 
  120 PRINT AT 21,0;n;" Sides";"      ";n*(n-3)/2;" diagonals"
  130 FOR t=1 TO n+1
  140 FOR q=t TO n+1
  150 PLOT x(t),y(t): DRAW x(q)-x(t),y(q)-y(t)
  160 NEXT q: NEXT t
  170 NEXT n