8-200.cpp

能判断二次函数是否有根

/*3208006411*/
#include<stdio.h>
#include<math.h>
float f(float x)
{float y;
y=x*x*x-5*x*x+16*x-80;
return(y);
}
float xpoint(float x1,float x2)
{
float y;
y=(x1+x2)/2;
return(y);
}
float root(float x1,float x2)
{
float x,y,y1;
y1=f(x1);
do
{
 x=xpoint(x1,x2);
 y=f(x);
 if(y*y1>0)
 {y1=y;
 x1=x;
 }
 else
   x2=x;
   }
while(fabs(y)>=1e-5);
return(x);
}
void main()
{float x1,x2,f1,f2,x;
do
{
printf("input x1,x2:\n");
 scanf("%f,%f",&x1,&x2);
  f1=f(x1);
  f2=f(x2);
  }
while(f1*f2>=0);
x=root(x1,x2);
printf("A root of the equation is %8.4f\n",x);
}