Triangular number is the amount of points that can fill equilateral triangle. 

Example: the number 6 is a triangular number because all sides of a triangle has the same amount of points.

```
Hint!
T(n) = n * (n + 1) / 2,
n - is the size of one side.
T(n) - is the triangular number.
```

Given a number 'T' from interval [1; 2147483646], find if it is triangular number or not.

Difficulty: introductory

Starter code:

def is_triangular(t):
	

Solutions:
def is_triangular(t):
    x = int((t*2)**0.5)
    return t == x*(x+1)/2

def is_triangular(t):
    return (8 * t + 1)**.5 % 1 == 0


def is_triangular(t):
    return (8 * t + 1) ** 0.5 % 1 == 0

def is_triangular(n):
    a = int((n * 2)**0.5)
    return n * 2 == a * (a + 1)


TRIANGULAR = { x*(x+1)/2 for x in range(10000) }

is_triangular = TRIANGULAR.__contains__


def is_triangular(t):
    return not (8 * t + 1)**.5 % 1

def is_triangular(n):
    return ((-1 + (1 + 8 * n)**.5) / 2) % 1 == 0


from math import sqrt
def is_triangular(t):
    n = (sqrt(8 * t + 1)-1)/2
    return False if n - int(n) > 0 else True

def is_triangular(t):
    return (-1 + (1 + 8 * t) ** 0.5)/2. == int((-1 + (1 + 8 * t) ** 0.5)/2.)


def is_triangular(t): return not (((1+8*t)**.5-1)/2%1)