title: ARRAYFORMULA with SUM OR SUMIF?
body: <p>I'm trying my to use <code>ARRAYFORMULA</code> with <code>SUM (or SUMIF?)</code>.  I basically want to lock <code>C1</code> and always <code>SUM</code> from <code>C1</code> down</p>
  
  <pre><code>=ARRAYFORMULA((SUM(C1:C2) + 1)&amp;":"&amp;(SUM(C1:C3)))         IN D3 is this
  =ARRAYFORMULA((SUM(C1:C3) + 1)&amp;":"&amp;(SUM(C1:C4)))         IN D4 is this
  </code></pre>
  
  <p>Here is sample sheet and below is visual.</p>
  
  <p>Col C is 50, 20, 16, etc.<br>
  Col D is 2:50, 51:70, 71:86, etc.</p>
  
  <p><a href="https://docs.google.com/spreadsheets/d/1DANMNEahYAoYBCQO1BsfXfUrgPj2mVWNKjn7VuYIIyI/edit#gid=0" rel="nofollow noreferrer">https://docs.google.com/spreadsheets/d/1DANMNEahYAoYBCQO1BsfXfUrgPj2mVWNKjn7VuYIIyI/edit#gid=0</a></p>
  
  <pre><code>units   desired_result
  50      2:50
  20      51:70
  16      71:86
  8       87:94
  2       95:96
  </code></pre>
  
  <p>If you could give a brief explanation on logic that'd be great.    Google's is confusing (as always) and Youtube is limited.</p>