C :

#include <stdio.h>
#include <math.h>
/* 定义f函数，计算题目中的函数f(x)=x3-5x2+15x-80 */
float f(float x) {
        float y;
        y = ((x - 5.0) * x + 16.0) * x - 80.0;
        return y;
}
/* 定义xpoint函数，计算弦与x轴的交点 */
float xpoint(float x1, float x2) {
        float y;
        y = (x1 * f(x2) - x2 * f(x1)) / (f(x2) - f(x1));
        return y;
}
/* 定义root函数，计算近似的根 */
float root(float x1, float x2) {
        float x, y, y1;
        y1 = f(x1);
        do {
                x = xpoint(x1, x2);
                y = f(x);
                /* 如果f(x)与f(x1)同符号 */
                if (y * y1 > 0) {
                        y1 = y;
                        x1 = x;
                } else {
                        x2 = x;
                }
        } while (fabs(y) >= 0.0001);
        return x;
}
int main() {
        float x1, x2, x;
        scanf("%f%f", &x1, &x2);
        x = root(x1, x2);
        printf("%.4f\n", x);
        return 0;
}

C++ :

#include <stdio.h>
#include <math.h>
/* 定义f函数，计算题目中的函数f(x)=x3-5x2+15x-80 */
float f(float x) {
	float y;
	y = ((x - 5.0) * x + 16.0) * x - 80.0;
	return y;
}
/* 定义xpoint函数，计算弦与x轴的交点 */
float xpoint(float x1, float x2) {
	float y;
	y = (x1 * f(x2) - x2 * f(x1)) / (f(x2) - f(x1));
	return y;
}
/* 定义root函数，计算近似的根 */
float root(float x1, float x2) {
	float x, y, y1;
	y1 = f(x1);
	do {
		x = xpoint(x1, x2);
		y = f(x);
		/* 如果f(x)与f(x1)同符号 */
		if (y * y1 > 0) {
			y1 = y;
			x1 = x;
		} else {
			x2 = x;
		}
	} while (fabs(y) >= 0.0001);
	return x;
}
int main() {
	float x1, x2, x;
	scanf("%f%f", &x1, &x2);
	x = root(x1, x2);
	printf("%.4f\n", x);
	return 0;
}

Pascal :

var 
  x,x1,x2:double;
  
// f(x)=x^3-5x^2+16x-80 
function f(x:double):double;
begin
  f:=((x-5)*x+16)*x-80;
end;

function xp(x1,x2:double):double;
begin
  xp:=(x1*f(x2)-x2*f(x1)) / (f(x2)-f(x1));
end;

function root(x1,x2:double):double;
var x,y,y1:double;
begin
  y1:=f(x1);
  repeat
    x:=xp(x1,x2);
    y:=f(x);
    if(y*y1>0) then begin //f(x)与f(x1)同号
      y1:=y; x1:=x;
    end else x2:=x; //否则x作为x2
  until abs(y)<1e-6;
  exit(x);
end;

begin
  readln(x1,x2);
  x:=root(x1,x2);
  writeln( x:0:4);
end.