c语言计算三角形面积，存在不懂知识

Question

include

include

double main()
{
double a,b,c,size,area;
a=3.1;
b=4.1;
c=5.1;
size=area(a,b,c);
printf("the size of this triangle is:%lf\n",size);
return 0;
}
double area(double a,double b,double c)
{
double area,s;
s=(a+b+c)/2;
area=sqrt(s*(s-a)*(s-b)*(s-c));
return area;
}

error C2064: term does not evaluate to a function这是错误信息，希望大神指导

Answer 1

double main()
->
int main()

在int main上面加上
double area(double a,double b,double c);

double a,b,c,size,area;
->
double a,b,c,size;

 # include <stdio.h>
# include <math.h>
double area(double a,double b,double c);
int main()
{
double a,b,c,size;
a=3.1;
b=4.1;
c=5.1;
size=area(a,b,c);
printf("the size of this triangle is:%lf\n",size);
return 0;
}
double area(double a,double b,double c)
{
double area,s;
s=(a+b+c)/2;
area=sqrt(s*(s-a)*(s-b)*(s-c));
return area;
}

the size of this triangle is:6.354173