梯形的面积公式：（上底+下底）*高/2

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#include<bits/stdc++.h>
using namespace std;
int main(){
    double a,b,c,a1,b1,c1;
    cin>>a>>b>>c;
    a1=a;b1=b;c1=c;
    if(a>b)swap(a,b);
    if(b>c)swap(b,c);
    if(a>b)swap(a,b);
    if(a+b<=c)printf("Area = %.1lf",(a1+b1)*c1/2);//注意，这里要用原来的底和高
    else printf("Perimetro = %.1lf",a+b+c);
}