1  REM  ****  HP BASIC PROGRAM LIBRARY  *******************************
2  REM
3  REM        POLYGN:   COMPUTES THE AREA ENCLOSED IN ANY POLYGON
4  REM
5  REM        36703 REV  A   10/73
6  REM
7  REM  ****  CONTRIBUTED PROGRAM  ************************************
10  REM POLYGN--COMPUTES THE AREA OF A POLYGON
20  PRINT "AFTER EACH ? TYPE THE X,Y COORDINATES OF POINTS ON THE"
30  PRINT "PERIMETER IN CLOCKWISE SEQUENCE."
40  PRINT "THE LAST POINT MUST BE THE SAME AS THE FIRST."
50  GOSUB 9000
60  PRINT "THE AREA IS";A1
70  STOP 
9000  REM  SUBROUTINE TO COMPUTE THE AREA OF A POLYGON GIVEN
9010  REM  ANY NUMBER OF POINTS ON THE PERIMETER IN CLOCKWISE ORDER.
9020  REM  THE LAST POINT ENTERED MUST EQUAL THE FIRST TO CAUSE A RETURN.
9030  LET A1=0
9040  INPUT X1,Y1
9050  INPUT X2,Y2
9060  LET D1=X2-X1
9070  LET D2=Y2-Y1
9080  INPUT X3,Y3
9090  IF X3=X1 AND Y3=Y1 THEN 9170
9100  LET D3=X3-X1
9110  LET D4=Y3-Y1
9120  LET A=(D2*D3-D1*D4)/2
9130  LET A1=A1+A
9140  LET D1=D3
9150  LET D2=D4
9160  GOTO 9080
9170  REM  A1 IS COMPUTED AREA.  IT MAY BE SCALED HERE IF NEEDED.
9180  RETURN 
9999  END 
