Saiyan
  • Saiyan
Help I don' t understand how to do this question! I will give medals
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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schrodinger
  • schrodinger
I got my questions answered at brainly.com in under 10 minutes. Go to brainly.com now for free help!
Saiyan
  • Saiyan
1 Attachment
Saiyan
  • Saiyan
I think it is b
IrishBoy123
  • IrishBoy123
\(S = 4 + 3 + 2 + .... + \ (-14) + \ (-15)\) but reverse it and: \(S = (-15) + (-14) + ... + \ 3 + \ 4\) so \(2S = S + S = \ ...???\)

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IrishBoy123
  • IrishBoy123
-15 + 4 = -11 -14 + 3 = -11 etc etc
Saiyan
  • Saiyan
yeah i tried that and i got -130
Saiyan
  • Saiyan
4+3+2+1+0-1-2-3-4-5-6-7-8-9-10-11-12-13-14-15-16
jdoe0001
  • jdoe0001
@Saiyan what would be the 1st term in the sequence?
Saiyan
  • Saiyan
what do u mean
IrishBoy123
  • IrishBoy123
-130 !!! you have 20 terms in the sequence, right?!?! as you're summing from n = 1 to 20
IrishBoy123
  • IrishBoy123
continue with this and you will get there
jdoe0001
  • jdoe0001
well, is a sequence , notice \(\large \bf \sum\limits_{n=1}^{20}\quad (5-n)\qquad thus\qquad a_1=?\)
jdoe0001
  • jdoe0001
\(\large a_1=first\ term=?\)
jdoe0001
  • jdoe0001
\(\large \begin{array}{llll} \sum\limits_{{\color{brown}{ n=1}}}^{20}\\ \ \uparrow \\ hint \end{array}\)
IrishBoy123
  • IrishBoy123
so that's 20 x (-11) / 2 yes?
Saiyan
  • Saiyan
yeah
jdoe0001
  • jdoe0001
ahemmm using \(\large S_n=\cfrac{n}{2}(a_1+a_n)\) yes

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