10 SCREEN 1
20 WINDOW (-10, 10)-(10, -10)
30 PRINT "Proof that the sum of angles in"
40 PRINT "a triangle equal 180"; CHR$(248)
50 LINE (-3, 0)-(2, 0)
60 LINE (-3, 0)-(0, 4)
70 LINE (0, 4)-(2, 0)
80 LINE (-5, 4)-(5, 4)
90 LOCATE 9, 18
100 PRINT "a"
110 LOCATE 9, 20
120 PRINT "c"
130 LOCATE 9, 22
140 PRINT "b"
150 LOCATE 12, 17
160 PRINT "a"
170 LOCATE 12, 22
180 PRINT "b"
190 LOCATE 7, 10
200 PRINT "Line parallel to base"
210 LOCATE 14, 1
220 PRINT "Angles a are congruent"
230 PRINT "because they are alternate interior."
240 PRINT "The same is true of the two b angles."
250 PRINT "Therefore the sum of a b and c is 180"; CHR$(248)