document.write( "Question 774396: each of my five digits is one more than the digit to its left.the sum of my digits is 20.what number am i ? \n" ); document.write( "
Algebra.Com's Answer #472231 by DrBeeee(684)\"\" \"About 
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What you are dealing with is the sum of the first n integers from 1 to n or let
\n" ); document.write( "(1) Sn = 1 + 2 + 3 + 4 + ... + n
\n" ); document.write( "You can figure out your answer by trial and error (the hard way).
\n" ); document.write( "(2) if n = 3; S3 = 1 + 2 + 3 = 6
\n" ); document.write( "(3) if n = 6; S6 = 1 + 2 + 3 + 4 + 5 + 6 = 21
\n" ); document.write( "You want the sum to be equal to 20 so start at 2 instead of 1 and get
\n" ); document.write( "(4) 20 = 2 + 3 + 4 + 5 + 6
\n" ); document.write( "Not too hard but not very mathematical. So let me give you a little lesson on sums
\n" ); document.write( "given by (1). The formula for (1) is
\n" ); document.write( "(5) Sn = n*(n+1)/2
\n" ); document.write( "Let's try it for n = 3,
\n" ); document.write( "(6) S3 = 3*(3+1)/2 or
\n" ); document.write( "(7) S3 = 3*4/2 or
\n" ); document.write( "(8) S3 = 6 as given by (2).
\n" ); document.write( "Let's try it for n = 6,
\n" ); document.write( "(9) S6 = 6*(6+1)/2 or
\n" ); document.write( "(10) S6 = 6*7/2 or
\n" ); document.write( "(11) S6 = 21 as given by (3).
\n" ); document.write( "How about the sum of 1 to 100? Want to add them up or use (5)? While you add I'll use (5) to get
\n" ); document.write( "(12) S100 = 100*101/2 or
\n" ); document.write( "(13) S100 = 5050
\n" ); document.write( "Easy isn't it? Please remember this formula, it will come in handy when you study probability.\r
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