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Thread: Induction to prove

  1. #1
    Junior Member
    Sep 2008

    Induction to prove

    Can someone please help me with the following 2 questions. I don't know how to type them as text. They are attached. Q.. 3 and 4 i need to use mathematical induction to prove them.

    Attached Thumbnails Attached Thumbnails Induction to prove-hw4.jpg  
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  2. #2
    Senior Member
    Nov 2007
    For the first question:
    If $\displaystyle n\ge 0$ is an integer, then $\displaystyle \sum_{j=0}^nj\cdot 2^j=(n-1)\cdot 2^{n+1}+2$
    We try it out for $\displaystyle n=0\text{ and } 1$. We suppose that $\displaystyle \sum_{j=0}^nj\cdot 2^j=(n-1)\cdot 2^{n+1}+2$ and prove for $\displaystyle n+1$:
    $\displaystyle \sum_{j=0}^{n+1}j\cdot 2^j=n\cdot 2^{n+2}+2\leftrightarrow \sum_{j=0}^nj\cdot 2^j+(n+1)\cdot 2^{n+1}=n\cdot 2^{n+1}\cdot 2+2=2^{n+1}\cdot\left(2n\right)+2$ which is by induction: $\displaystyle \left((n-1)\cdot 2^{n+1}+2\right)+(n+1)\cdot 2^{n+1}=2^{n+1}\cdot\left(2n\right)+2\leftrightarr ow2^{n+1}\left(n-1+n+1\right)=2^{n+1}\cdot\left(2n\right)$ QED.
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