float dienTichTG(float a, float b, float c);
float a, b, c, S;
printf("nhap do dai 3 canh tam giac: ");
scanf_s("%f %f %f", &a, &b, &c);
S = dienTichTG(a, b, c);
if (S != -1)
	printf("Dien tich tam giac la: %.2f\n", S);
break;
float dienTichTG(float a, float b, float c)
{
	float p, S;
	if (a + b <= c || a + c <= b || b + c <= a) {
		printf("Ba canh nay khong tao thanh tam giac!\n");
		return -1;
	}

	p = (a + b + c) / 2;
	S = sqrt(p * (p - a) * (p - b) * (p - c));

	return S;
}