title: check if a number is triangular
body: <p>Hello I am trying to figure out of a number is triangular so I started writing my logical train of thought and ran into a little problem with my code I get an error that states &quot;TypeError: 'int' object is not callable&quot; any help is most appreciated.
  here is what my code looks like that is causing this issue:</p>
  <pre><code>x = int(input(&quot;Please enter an int: &quot;))
  i = x(x+1)/2
  l = []
  l.append(i)
  print(i)
  </code></pre>