c语言 求一元二次方程的根的问题！！为什么输入1 2 1 还是输出不同根，=0的那个没用了？？？？？、求纠错

# include<stdio.h>
# include<math.h>
int main()
{
double a,b,c,p,q,disc,x1,x2;
scanf("%lf%lf%lf",&a,&b,&c);
disc=b*b-4*a*c;
if(disc<0)
printf("This equation hasn't real roots!\n");
else
if(disc=0)
{
p=-b/(2*a);
x1=x2=p;
printf("This equation has two identical roots:\nx1=x2=%5.4f\n",x1);
}
else
{
p=-b/(2*a);
q=sqrt(disc)/(2*a);
x1=p+q;x2=p-q;
printf("This equation has two different roots:\nx1=%5.4f\nx2=%5.4f\n",x1,x2);
}
return 0;
}