5 ONERRORGOTO900
20 MODE8:DIM:a=0:b=0:h=0:c$=CHR$(5)
30 CLS:PRINT"(a+b)h/2 a:b:base h:height";
40 s$="a":z=a:GOSUB100:a=z
50 s$="b":z=b:GOSUB100:b=z
60 s$="h":z=h:GOSUB100:h=z
70 LOCATE0,1:PRINTc$;"Area =";(a+b)*h/2;:z$=INPUT$(1,@):GOTO40
100 LOCATE0,1:PRINTc$;s$;"=";z;"?";:INPUT@42;z:LOCATE0,0:IFz<0 THEN100 ELSERETURN
900 IFERR=1 THENCLS:ONERRORGOTO0
910 IFERL=100 THENRESUME100
920 LOCATE0,1:PRINTc$;"not found";:z$=INPUT$(1,@):RESUME30