Display an introductory message: this program calculates the area of a triangle use the getdouble function to get the lengths (between 1 and 100) of the three sides from the user. this will require three calls to getdouble. use the calcarea function to get the area of the triangle. if calcarea returns a negative value for the area, then that indicates the triangle is invalid. if the triangle is invalid, then display an error message; otherwise display the area with one digit after the decimal point.