10 CLS 3
20 SCREEN 3
30 X0=100:Y0=200:Z=70
40 LINE (X0,Y0-280)-(X0,Y0+150),7
50 LINE (X0,Y0)-(X0+400,Y0),7
55 LINE (X0+180,Y0-5)-(X0+180,Y0+5)
58 LINE (X0+360,Y0-5)-(X0+360,Y0+5)
60 A=0.2   :  '←この値を変更する
100 FOR I=1 TO 360
110 THETA=I*3.141592#/180
120 Y=(COS(THETA)+1/A*(SQR(1-A^2*SIN(THETA)^2)))-1/A
130 IF I=1 THEN 140 ELSE 150
140 PSET (X0+I,Y0-Y*Z),7
150 LINE -(X0+I,Y0-Y*Z),7
160 NEXT I
170 '
180 WIDTH 80,25
190 LOCATE 8,4:PRINT"Xp/r"
200 LOCATE 11,12:PRINT"0"
210 LOCATE 34,11:PRINT"π"
220 LOCATE 56,13:PRINT"2π"