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Mimi_x3 Group Title

Prove: \[\binom{n-1}{k-1} +\binom{n-2}{k-1} +\binom{n-3}{k-1} +....+\binom{k-1}{k-1} =\binom{n}{k}\]

  • 2 years ago
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  1. satellite73 Group Title
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    i will guess induction

    • 2 years ago
  2. Mimi_x3 Group Title
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    induction? is there by chance another method?

    • 2 years ago
  3. satellite73 Group Title
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    maybe a thinking method, but i can't imagine there is another way, since you are making a statement that says essentially "for all \(n\in \mathbb{N}\) this formula holds"

    • 2 years ago
  4. satellite73 Group Title
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    the last part will be some annoying algebra with factorials, but since you have a sum with \(n-k\) terms you cannot add them i in a formula that i can think of. i think the induction step will be enough arithmetic as it is

    • 2 years ago
  5. satellite73 Group Title
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    for base case it is probably easiest to start at \(n=2\)

    • 2 years ago
  6. Mimi_x3 Group Title
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    can it be a gemoetric progression?

    • 2 years ago
  7. Mimi_x3 Group Title
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    geometric*

    • 2 years ago
  8. satellite73 Group Title
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    maybe, lets try it with numbers , say \(n=5, k=3\) then \(\dbinom{2}{2}=1,\dbinom{3}{2}=3, \dbinom{4}{2}=6\) no, not geometric

    • 2 years ago
  9. satellite73 Group Title
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    there is a think a way to do it by thinking, however it is almost like saying \[\dbinom{n}{k}=\dbinom{n-1}{k}+\dbinom{n-1}{k-1}\]

    • 2 years ago
  10. Mimi_x3 Group Title
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    hm..so induction is the only method? but i havent learnt induction to prove these kind of things..

    • 2 years ago
  11. joemath314159 Group Title
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    You can just keep using:\[\left(\begin{matrix}n \\ k\end{matrix}\right)=\left(\begin{matrix}n-1 \\ k\end{matrix}\right)+\left(\begin{matrix}n-1 \\ k-1\end{matrix}\right)\]over and over again. In a sense, its an inductive argument. Using it once gives you the above statement. Notice the second part of the sum is something you want, but not the first part. So use that idea on the first one again:\[\left(\begin{matrix}n-1 \\ k\end{matrix}\right)=\left(\begin{matrix}n-2 \\ k\end{matrix}\right)+\left(\begin{matrix}n-2 \\ k-1\end{matrix}\right)\]So altogether we have:\[\left(\begin{matrix}n \\ k\end{matrix}\right)=\left(\begin{matrix}n-2 \\ k\end{matrix}\right)+\left(\begin{matrix}n-2 \\ k-1\end{matrix}\right)+\left(\begin{matrix}n-1 \\ k-1\end{matrix}\right)\]Again, the last two parts of the sum are what you want, but the first isnt, so keep repeating this process (this is the inductive reasoning). You do this all the way until the n-2 on the top becomes k-1.

    • 2 years ago
  12. Mimi_x3 Group Title
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    well, thank you!

    • 2 years ago
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