Triangular number

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

Code:

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#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;
}

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

int is 32 bits long (UINT_MAX = 4294967295)
long long is 64 bits long
double is 53. Try:

Code:

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bool is_triangular( unsigned long long t )
{
  unsigned long long delta = 1ULL+8ULL*t;
  long double x = (-1.0L + sqrtl(delta)) / 2.0L;
  return floorl( x ) == x;
}

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Thank you for answer