int main（）{ double a，b，c，area；printf（“Enter the sides of the triangle：”）；scanf（“%lf%lf%lf”，&；a，&；b，&；c）；if（a+b>；c&；&；a+c>；b&；&；b+c>；a）{ area=0.5*sqrt（a* b* c）；printf（“The triangle is valid.\n”）；printf（“The area...