Fortran ~ Bisection Method

• This method is used for finding an approximate solution to an equation f(x)=0 to the desired degree of accuracy. 
• This method is based on a concept which states that if a function f(x) is continuous in the closed interval a<=x<=b and f(a), f(b) are of opposite signs, 
• then there must exist at least one real root between x=a and x=b.
• F(X) =2.0*X**3-5.0*X-2.15

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Write a program to demonstrate the application of the Bisection method to locate the root of the equation.?

PROGRAM BISECT
C
C        PROGRAM TO LOCATE A ROOT OF EQUATION USING BISECTION METHOD
C
C         GIVEN EQUATION
            F(X) =2.0*X**3-5.0*X-2.15
C
100      WRITE (*,*)’ENTER YOUR ESTIMATE OF ROOT (LOWER LIMIT, UPPER LIMIT)’
            READ (*,*) XL, XR
C         FIND IF ESTIMATES ARE CORRECT
           CHK=F(XL)*F(XR)
C         ENTER DATA AGAIN IF ESTIMATES DO NOT BRACKET THE ROOT
           IF (CHK.GT.0.0) GOTO 100
           I=0
400     I=I+1
C        FIND ROOT POSITION USING ARITHMETIC MEAN
           XM=(XL+XR)/2.0
           CHK=ABS (F (XM))
C        EXIT LOOP IF ROOT FOUND
          IF (CHK.LE.0.0001) GOTO 200
C        EXIT LOOPS IF UNABLE TO FIND ROOT EVEN AFTER 100+ CYCLES
           IF (I.GT.100) GOTO 300
C        GET NEW LOWER AND UPPER LIMITS FOR NEXT CYCLE
           CHK=F(XL)*F(XM)
           IF(CHK.LT.0.0)THEN
           XR=XM
          ELSE
           XL=XM
           ENDIF
C        START NEXT CYCLE
           GOTO 400
200    WRITE (*,*)’ROOT FOUND AT X=’, XM
           STOP
300    WRITE (*,*)’ROOT NOT FOUND. PROGRAM TERMINATED.’
           STOP
           END

#OUTPUT

ENTER YOUR ESTIMATE OF ROOT (LOWER LIMIT, UPPER LIMIT)
2 4
ROOT FOUNDATION AT X = 2.5815153