HDU 1071 The area（数学积分）

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思路：ax^2+bx+c=y;
kx+m=y;
∫（ax^2+bx+c-kx-m）dx；x2<=x<=x3;
用x1,y1,x2,y2,x3,y3表示里面的未知量。

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int t;cin>>t;
    while(t--)
    {
        double x1,y1;cin>>x1>>y1;
        double x2,y2;cin>>x2>>y2;
        double x3,y3;cin>>x3>>y3;
        double a,b,c,k,m;
        a=((y1-y2)/(x1-x2)-(y2-y3)/(x2-x3))/(x1-x3);
        b=(y1-y2)/(x1-x2)-a*(x1+x2);
        c=y1-a*x1*x1-b*x1;
        k=(y3-y2)/(x3-x2);
        m=y3-k*x3;
        double res1=a/3*pow(x3,3)+(b-k)/2*pow(x3,2)+(c-m)*x3;
        double res2=a/3*pow(x2,3)+(b-k)/2*pow(x2,2)+(c-m)*x2;
        double res=res1-res2;
        printf("%.2lf\n",res);
    }
    return 0;
}