4.　初等幾何学の作図例題

10 REM 《三角形の傍心定理》
20 DEF2PT P: DEF2ED E: DEF2CR C: DEF2LN L
30 CLG
40 LET PA,PB,PC=-75,-50,75,-50,25,100
50 EAB=PA @ PB: EBC=PB @ PC: ECA=PC @ PA
60 LAB=EAB: LBC=EBC: LCA=ECA
70 ETAB=EAB/ABS(EAB):ETBC=EBC/ABS(EBC)
71 ETCA=ECA/ABS(ECA)
80 REM ==== compute angle bisector LA =======
90 ETA=ETAB+ETCA: ETA=PA: LA=ETA: 
100 REM ==== compute angle bisector LB =======
110 ETB=ETAB+ETBC: ETB=PB: LB=ETB
120 REM ==== compute angle bisector LC =======
130 ETC=ETBC+ETCA: ETC=PC: LC=ETC
140 REM ==================================
150 PRA=LB&LC: PRB=LC&LA: PRC=LA&LB
160 RA=DIS(PRA,EBC): RB=DIS(PRB,ECA)
161 RC=DIS(PRC,EAB)
170 LET CRA=PRA,RA: CRA=CRA
180 LET CRB=PRB,RB: CRB=CRB
190 LET CRC=PRC,RC: CRC=CRC
200 DPTEXT -50,250,"三角形の傍心"
210 LET X,Y=PA: DPTEXT X,Y,"A"
220 LET X,Y=PB: DPTEXT X,Y,"B"
230 LET X,Y=PC: DPTEXT X,Y,"C"
240 LET X,Y=PRA: DPTEXT X,Y,"PRA"
250 LET X,Y=PRB: DPTEXT X,Y,"PRB"
260 LET X,Y=PRC: DPTEXT X,Y,"PRC"

図4.4　三角形の傍心を求める