5 ONERRORGOTO500
10 MODE8:DIM:ERASEa:DIMa(4):a$="a b r x1y1":c$=CHR$(5)
20 CLS:PRINT"(x-a)ý+(y-b)ý=rý,(x1,y1)";
30 FORi=0 TO4
40 LOCATE0,1:PRINTc$;MID$(a$,1+2*i,(SGN(i-2.5)+1)/2+1);"=";a(i);"?";:INPUT@42;a(i):LOCATE0,0
45 IFi=2 AND a(2)<0 THEN40
50 NEXT:LOCATE0,1:PRINTc$;
60 w=(a(3)-a(0))^2+(a(4)-a(1))^2-a(2)^2:IFw<0 THEN100 ELSEPRINTc$;"l:length =";SQR(w);:w$=INPUT$(1,@):GOTO30
100 LOCATE0,1:PRINTc$;"not found";:w$=INPUT$(1,@):GOTO20
500 IFERR=1 THENCLS:ONERRORGOTO0
510 IFERL=40 THENRESUME40
520 RESUME100