4.　初等幾何学の作図例題

10 REM   三角形の内接円を求める
20 REM === 角の二等分線を求める ===
30 CLG
40 DEF2PT P : DEF2ED E : DEF2CR C : DEF2LN L   
50 LET PA,PB,PC=-200,-200,200,-200,100,200
60 EAB=PA @ PB : EAB1=EAB/ABS(EAB): EAB1=PA
70 EAC=PA @ PC : EAC1=EAC/ABS(EAC): EAC1=PA
80 LA=EAC1+EAB1 : REM bisector thru A
90 EBA=PB @ PA : EBA1=EBA/ABS(EBA): EBA1=PB
100 EBC=PB @ PC : EBC1=EBC/ABS(EBC): EBC1=PB
110 LB=EBA1+EBC1 : REM bisector thru B
120 PI=LA & LB : RI=DIS(PI,EAB) : LET CI=PI,RI 
130 CI=CI : REM inscribed circle drawing 
140 DPTEXT -100,250,"三角形の内接円"
150 LET X,Y=PA: DPTEXT X,Y,"A"
160 LET X,Y=PB: DPTEXT X,Y,"B"
170 LET X,Y=PC: DPTEXT X,Y,"C"
180 LET X,Y=PI: DPTEXT X,Y,"PI"

図4.3　三角形の内接円を求める