PROGRAM Triangle
IMPLICIT NONE

REAL :: a, b, c, Area

PRINT *, 'Welcome, please enter the&
          &lengths of the 3 sides.'
READ *, a, b, c
PRINT *, 'Triangle''s area:  ', Area(a,b,c)
END PROGRAM Triangle

FUNCTION Area(x,y,z)
  IMPLICIT NONE
  REAL :: Area            ! function type
  REAL, INTENT( IN ) :: x, y, z
  REAL :: theta, height

  theta = ACOS((x**2+y**2-z**2)/(2.0*x*y))
  height = x*SIN(theta); Area = 0.5*y*height
END FUNCTION Area