      REAL(8) :: x,t,x0=0,t0=1,te=11,h
      REAL(8) :: rk1,rk2,f
      INTEGER :: n=1000,i

      h=(te-t0)/n

      t=t0
      x=x0
      DO i=1,n
         CALL SUB(x,t,f)
         rk1=h*f
         CALL SUB(x+rk1/2,t+h/2,f)
         rk2=h*f
         x=x+rk2
         t=t+h
      END DO

      PRINT *,'t=',t,'  x=',x,'  60-x=',60-x

      END
!--------------------------------------
      SUBROUTINE SUB(x,t,f)

      REAL(8) :: x,t,f

      f=(x+SQRT(x*x+t*t))/t

      END



      
      
