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The Most Important Sum

The Most Important Sum. 1+2+3+…+(n-1)+n =. n(n+1) 2. Applications:. n people; how many handshakes? (equivalent: n cities to directly connect with phone cables; how many cables needed?) Arranging n cards in order: how many compares? Appending to a list of names: how many steps?

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The Most Important Sum

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  1. The Most Important Sum 1+2+3+…+(n-1)+n = n(n+1) 2

  2. Applications: • n people; how many handshakes? • (equivalent: n cities to directly connect with phone cables; how many cables needed?) • Arranging n cards in order: how many compares? • Appending to a list of names: how many steps? • Keeping students busy (anecdote: Gauss)

  3. Three derivations • Gauss' approach: 1 + 2 + 3 + … + (n-2) + (n-1) + n = S n +(n-1)+(n-2)+ … + 3 + 2 + 1 = S (n+1)+(n+1)+(n+1)+…+(n+1) + (n+1) + (n+1) = 2S n(n+1) = S 2

  4. Three Derivations (part 2) 1 2 3 . . . n-1 n . . . . . . . . . 1 2 3 n-1 n

  5. Three Derivations (part 2) . . . 1 2 3 . . . n-1 n . . . . . . . . . . . . n(n+1) = S 2 . . . 1 2 3 n-1 n

  6. Three Derivations (part 3) Count the ways to create a handshake: • Choose the 1st person (n choices) • Choose the 2nd person (n-1 remaining choices(That's n(n-1) ways to choose one then the other.) • But this overcounts: "Choose Alice then Bob" and "Choose Bob then Alice" are the same handshake; divide by 2 to correct. S = n(n+1) 2

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