Well, what you said only works if it's an even number. For an odd number, I get to draw another diagram...JOY! :-D<br><br>1 2 3<br><br>+ + +<br><br>? 5 4<br>=========<br>? 7 7<br><br>The equation for and odd range is this:<br><br>((range+2)*((range/2)-.5))+1<br><br>The sum of the pair (5 and 2, 3 and 4, etc.) is range+2 all the way across. To get the number of pairs, you take half of the range rounded down. We know it will end in .5 because the range is odd, so we just subtract .5. We multiply them just like we did 50 and 101. Now add 1 to compensate for that 1 left after the pairing. For example, 105 is ((105+2)*((105/2)-.5))+1, or<br>((107)*((52.5)-.5))+1, or<br>(107*52)+1, or<br>5564+1, or<br>5565.<br><br>There, I showed my work!<br><br>EVEN: (range*2)*(range+1)<br>ODD: ((range+2)*((range/2)-.5))+1