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Saiyan

  • one year ago

Help I don' t understand how to do this question! I will give medals

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  1. Saiyan
    • one year ago
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  2. Saiyan
    • one year ago
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    I think it is b

  3. IrishBoy123
    • one year ago
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    \(S = 4 + 3 + 2 + .... + \ (-14) + \ (-15)\) but reverse it and: \(S = (-15) + (-14) + ... + \ 3 + \ 4\) so \(2S = S + S = \ ...???\)

  4. IrishBoy123
    • one year ago
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    -15 + 4 = -11 -14 + 3 = -11 etc etc

  5. Saiyan
    • one year ago
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    yeah i tried that and i got -130

  6. Saiyan
    • one year ago
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    4+3+2+1+0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16

  7. jdoe0001
    • one year ago
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    @Saiyan what would be the 1st term in the sequence?

  8. Saiyan
    • one year ago
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    what do u mean

  9. IrishBoy123
    • one year ago
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    -130 !!! you have 20 terms in the sequence, right?!?! as you're summing from n = 1 to 20

  10. IrishBoy123
    • one year ago
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    continue with this and you will get there

  11. jdoe0001
    • one year ago
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    well, is a sequence , notice \(\large \bf \sum\limits_{n=1}^{20}\quad (5-n)\qquad thus\qquad a_1=?\)

  12. jdoe0001
    • one year ago
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    \(\large a_1=first\ term=?\)

  13. jdoe0001
    • one year ago
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    \(\large \begin{array}{llll} \sum\limits_{{\color{brown}{ n=1}}}^{20}\\ \ \uparrow \\ hint \end{array}\)

  14. IrishBoy123
    • one year ago
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    so that's 20 x (-11) / 2 yes?

  15. Saiyan
    • one year ago
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    yeah

  16. jdoe0001
    • one year ago
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    ahemmm using \(\large S_n=\cfrac{n}{2}(a_1+a_n)\) yes

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