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history_source/信息学奥赛一本通/1.2.6.6.cpp

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+#include<cstdio>
+#include<cmath>
+using namespace std;
+struct Point
+{
+	double x,y;
+};
+double lenth(Point A,Point B)
+{
+	double a=abs(A.x-B.x),b=abs(A.y-B.y),c=sqrt(a*a+b*b);
+	return c;
+}
+int main()
+{
+	Point A,B,C;
+	scanf("%lf%lf%lf%lf%lf%lf",&A.x,&A.y,&B.x,&B.y,&C.x,&C.y);
+	double a,b,c;
+	a=lenth(A,B);
+	b=lenth(B,C);
+	c=lenth(C,A);
+	double p=(a+b+c)/2.0;
+	double ans=sqrt(p*(p-a)*(p-b)*(p-c));
+	printf("%.2lf\n",ans);
+	return 0;
+}