Thread: Cubic equation program not giving me right answers

Im getting the wrong roots when running the program. The maths the program is based on is in the attatched image.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define pi 3.14159265

int sign(float s);

int main()
{
	float a1, a2, a3, x1, x2, x3, s, q, th, s_sq, q_cub, a1_sq, a1_cub, s_abs;	


	printf("The cubic equation has the form x^3 + a1x^2 + a2x + a3 = 0\n");
	
	printf("Enter a1: ");
	scanf("%f", &a1);
	
	printf("\nEnter a2: ");
	scanf("%f", &a2);

	printf("\nEnter a3: ");
	scanf("%f", &a3);

	a1_cub = pow(a1, 3);
	a1_sq = pow(a1, 2);

	/* calculate q and s */

	q = ( a1_sq - (3 * a2) ) / 9;
	
	s = ( (2 * a1_cub) - (9 * (a1*a2) ) + (27 * a3) ) / 54;

	/* get square / cube of s and q  */

	s_sq = pow(s, 2);
	q_cub = pow(q, 3);

	s_abs = fabs(s);


	/* ****** */

	th = acos( (s/(sqrt(q_cub)) )); /* calculate theta */

	if( (q_cub - s_sq) >= 0)	/* cubic equation has 3 roots */
	{
		x1 = ((-2)*(sqrt(q))*(th/3)) - (a1/3);
		
		x2 = ((-2)*(sqrt(q))*((th + (2*pi))/3)) - (a1/3);

		x3 = ((-2)*(sqrt(q))*((th + (4*pi))/3)) - (a1/3);

		printf("\nx1 = %f\nx2 = %f\nx3 = %f\n", x1, x2, x3);

	}
	else if( (s_sq - q_cub) > 0 ) /* cubic equation has 1 root */
	{
		
		float part1 = sqrt(s_sq - q_cub) + s_abs;

		x1 = -(sign(s)) * ( (pow(part1, (1/3))) + (q/(pow(part1, (1/3)))) ) - (a3/3);
		                                                                                                                                           
		printf("\nx1 = %f", x1);
		
	}
	else
	{
		printf("something crap happened\n");
	}


	system("pause");
	
	return 0;		
}

int sign(float s)
{
	if(s > 0)
	{
		return 1;
	}
	else if(s < 0)
	{
		return -1;
	}
	
}