program newt2
c
      integer out
      real x
      external sqr2
      common /inout/ out
c
      out = 6
      write(out,*) ' newton-s method'
10    write(out,103)
      read(*, 104) x
      if (x .lt. -19.0) goto 99
      call newton(x, sqr2)
      write(out, 102) x
      goto 10
99    stop
102   format(/' the square root of 2 is ', f8.5)
103   format(' first guess? ' )
104   format(e10.0)
      end
      subroutine sqr2(x, fx, dfx)
c
c -- the square root of 2
c
      real x, fx, dfx
c
      fx = x * x - 2
      dfx = 2 * x
      return
      end
      subroutine newton(x, func)
c
c -- the solution of f(x) = 0 by newton-s method
c
      integer out
      real x, fx, dfx, x1, dx, tol
      common /inout/ out
      data tol/1.0e-6/
c
      write(out, 101)
10     x1 = x
       call func(x, fx, dfx)
       dx = fx / dfx
       x = x1 - dx
       write(out, 102) x1, fx, dfx
      if (abs(dx) .gt. abs(x * tol)) goto 10
      return
101   format('     x            fx      dfx')
102   format(1x, 0pf13.5, 3x, 1p2e12.4)
      end