87.c

计算概论阅读材料

#include <stdio.h>
#include <math.h>
double efroot(f,x1,x2) double x1,x2,(*f)(double x);
{
	double x,y1,y2,y;
	y1=(*f)(x1);
	y2=(*f)(x2);
	if(fabs(y1)<1e-6) return x1;
	if(fabs(y2)<1e-6) return x2;
	if(y1*y2>=0){
		printf("same sign");return 0;
	}
	do{
		x=0.5*(x1+x2);
		y=(*f)(x);
		if(y*y1<0){x2=x;y2=y;}
		else {x1=x;	y1=y;}
		}while(fabs(x1-x2)>1e-6&&fabs(y)>1e-6);
	return x;
}
double fpoly1(double x){ return (x-1)*(x-2)*(x-3);}
double fpoly2(double x){ return (x+1)*(x+2)*(x+3);}
void main(void)
{
	double x;
	x=efroot(fpoly1,4.5,0.5);
	printf("x=%lf\n",x);
	x=efroot(fpoly2,-1.5,-2.5);
	printf("x=%lf\n",x);
}