Triangular number

[gawiellus]

Triangular number is the amount of point that can fill equilateral triangle. For example number 6 is triangular number because all sides of triangle has the same amount of point. The problem for this task is to solve equation T(n) = n * (n + 1) / 2. t(n) is given we are searching n. My code below

#include <stdbool.h>


bool is_triangular(int t) {
unsigned long long int delta = 1+8*t;  
double x=(-1+sqrt(delta))/2.0;
    if(floor(x)==x)
      return true;
return false;
}

It gives me proer solutions except t==458000245 and for t==453351216. I don't know where is the problem.