5 ONERRORGOTO500
10 MODE8:DIM:a=0:c=0:c$=CHR$(5)
20 CLS:PRINT"Angle(é) - y=ax+b,y=cx+d";
30 s$="a":z=a:GOSUB100:a=z
40 s$="c":z=c:GOSUB100:c=z:LOCATE0,1
60 IFa=c THENPRINTc$;"Parallel"; ELSEIFa*c=-1 THENPRINTc$;"Right angle"; ELSEPRINTc$;"é =";ATN((c-a)/(1+c*a));
70 w$=INPUT$(1,@):GOTO30
100 LOCATE0,1:PRINTc$;s$;"=";z;"?";:INPUT@42;z:LOCATE0,0:RETURN
500 IFERR=1 THENCLS:ONERRORGOTO0
510 IFERL=100 THENRESUME100
520 LOCATE0,1:PRINTc$;"not found";:z$=INPUT$(1,@):RESUME20