void rectan(){

double a,b,c, square;

double tmp;

char max;

scanf("%lf%lf%lf",&a,&b,&c);

if( a == b || b==c || a ==c){//等腰三角形一定為銳角（包含等邊）

printf("銳角三角形\n");

return;

}

//將角C選為最大角，若輸入不是此交換使C最大

max = a>b?(tmp = a,"a"):(tmp = b,"b");

max = tmp>c?max:(tmp = c,"c");

if(max != "c"){

tmp = a;

a = c;

c = tmp;

}

//兩銳角所對的邊的平方和

square = a*a + b*b;

//由余弦定理：cosC = (a*a + b*b - c*c)/(2ab) ， 判斷最大角C的正負號

//就是判斷 a*a+b*b - c*c的正負號

if( c*c == square)//相等是直角

printf("直角三角形\n");

else if(c*c

printf("銳角三角形\n");

else//相減為負數為鈍角

printf("銳角三角形\n");

}