hdu 1215 The area

算抛物线和直线解析式，简单积分

#include <iostream>
#include<cstdio>
using namespace std;
double  p2x,p2y,p1x,p1y,p3x,p3y;
double  a,b,c,a1,b1;
double  ans;
void solve1()
{
    //求直线
    a1 = (p3y-p2y)/(p3x-p2x);
    b1 = (p2y - p2x*a1);
    //求抛物线解析式
    double  t1,t2,q1,q2,p1,p2;
    t1 = p2x*p2x - p1x*p1x;
    t2 = p1x*p1x - p3x*p3x;
    p1 = p2x-p1x;
    p2 = p1x-p3x;
    q1 = p2y-p1y;
    q2 = p1y-p3y;
    a = (q1*p2 - q2*p1)/(t1*p2-t2*p1);
    b = (q1-a*t1)/p1;
    c = p2y - (a*p2x*p2x + b*p2x);
    //求差方程解析式
    b = b - a1;
    c = c - b1;
    //求积分
    a = a/3;
    b = b/2;
    ans = (p2x*p2x*p2x*a) + (p2x*p2x*b) + (p2x*c);
    ans -=(p3x*p3x*p3x*a) + (p3x*p3x*b) + (p3x*c);
    ans = ans > 0?ans:-ans;
}
int main()
{
    int cas;
    cin  >> cas;
    while(cas --){
        cin >> p1x>>p1y>>p2x>>p2y>>p3x>>p3y;
        solve1();
        printf("%0.2f\n",ans);
    }
    return 0;
}