求解ax^2+bx+c=0的根

#include<stdio.h>
#include<math.h>  //调用求平方根的函数sqrt
int main()
{
 double a,b,c,x1,x2,p,q,r;
 printf("please input three number:\n");
 scanf("%lf%lf%lf",&a,&b,&c);
 r=b*b-4*a*c;
 
 if(r<0)
 {
  printf("real root does not exit\n");


 }
 else
 {
  p=-b/(2.0*a);
  q=sqrt(r)/(2*a);
  x1=p+q;
  x2=p-q;
  printf("the real root is:\nx1=%7.2f\nx2=&7.2f",x1,x2);
 }
 return 0;
}