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DDT

  • 3 years ago

Find minimum of : \sum_{k=1}^{n}a_{k}^{2}+\left(\sum_{k=1}^n a_k\right)^2. My teacher told me that I shoul use Cauchy-Schwarz inequality. Any tips?

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  1. henpen
    • 3 years ago
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    \[ \sum_{k=1}^{n}a_{k}^{2}+\left(\sum_{k=1}^n a_k\right)^2\] \[\left(\sum_{k=1}^n a\right)^2=\sum_{k=1}^n a^3\] So your question boils down to \[\sum_{k=1}^n a^2+ a^3=\sum_{k=1}^n a^2(a+1)\]

  2. henpen
    • 3 years ago
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    http://users.tru.eastlink.ca/~brsears/math/oldprob.htm#s32

  3. henpen
    • 3 years ago
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    The link is the ()^2=(^3) proof

  4. henpen
    • 3 years ago
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    Although that's not Cauchy Schwartz.

  5. DDT
    • 3 years ago
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    I didn't realise that task has another assumption :\[\sum_{k=1}^{n}p_ka_k=1\]

  6. henpen
    • 3 years ago
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    Sorry, I forgot that a1 does not necessarily equal 1, a2=2 etc. forget what I've told you so far.

  7. henpen
    • 3 years ago
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    http://en.wikipedia.org/wiki/Cauchy%E2%80%93Schwarz_inequality#Statement_of_the_inequality you want the final formula of this section

  8. henpen
    • 3 years ago
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    Or do you think a1=1, a2=2 etc in this problem?

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