Ive written the program below to determine types of triangle from their side lengths. The problem is that it cannot seem to determine when its a right angled triangle. It gives either acute (all angles less than 90 degrees) or obtuse (one angle greater than 90 degrees).

The side lengths 3, 4 and 5 will give one interior angle of exactly 90 degrees, yet the program produces 89.99999 etc. Im sure its a rounding problem, but can anyone help me rewrite the program to cure it?

Code:#include <stdio.h> #include <math.h> int main() { char another='Y'; double a,b,c,A,B,C; printf("This is a program to categorise triangles by side length and angle size\n"); while(another == 'Y' || another == 'y') { printf("Enter 3 real numbers for each side of the triangle\n"); scanf("%lf%lf%lf",&a,&b,&c); if(a>0 && b>0 && c>0) { A = (b*b + c*c - a*a)/(2*b*c); /* calculates the value of cosA */ B = (a*a + c*c - b*b)/(2*a*c); C = (a*a + b*b - c*c)/(2*a*b); /* checks triangles are possible */ if((A>-1 && A<1) && (B>-1 && B<1) && (C>-1 && C<1)) { printf("The triangle you have input has side lengths %lf, %lf, %lf\n\n",a,b,c); printf("Angle A = %lf radians",acos(A)); printf(" or %lf degrees\n",acos(A)*(180/3.1415927)); printf("Angle B = %lf radians",acos(B)); printf(" or %lf degrees\n",acos(B)*(180/3.1415927)); printf("Angle C = %lf radians",acos(C)); printf(" or %lf degrees\n",acos(C)*(180/3.1415927)); if(a==b && b==c && c==a)printf("\nTriangle is equilateral"); if(a!=b && b!=c && c!=a)printf("\nTriangle is scalene"); if((a==b && c!=a) || (a==c && a!=b) || (b==c && b!=a))printf("\nTriangle is isoceles"); if(acos(A)==acos(0)||acos(B)==acos(0)||acos(C)==acos(0))printf(" and right-angled\n\n"); if(acos(A)>acos(0)||acos(B)>acos(0)||acos(C)>acos(0))printf(" and obtuse\n\n"); else if(acos(A)<acos(0)&&acos(B)<acos(0)&&acos(C)<acos(0))printf(" and acute\n\n"); } else printf("Triangle is impossible!\n\n"); } else printf("Input is invalid, lengths must be real numbers\n\n"); printf("Would you like to categorise another triangle?(Y or N)"); another=getchar(); while (another !='Y'&&another !='y'&&another !='n'&&another !='N') another=getchar(); } printf("\nThis program is now ending\n\n"); return 0; }