anonymous
  • anonymous
Find the sum of the arithmetic sequence. -10, -7, -4, -1, 2, 5, 8 11 -70 0 -7
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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schrodinger
  • schrodinger
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anonymous
  • anonymous
@misssunshinexxoxo
anonymous
  • anonymous
@peachpi
anonymous
  • anonymous
Goshhh cant you add them just add them you get -7

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anonymous
  • anonymous
oh so all i do is add?
anonymous
  • anonymous
that's what I'd do since they're listed
anonymous
  • anonymous
what if they give me only 4 terms and tell me to find the sum of the first 9
anonymous
  • anonymous
nevermind all you do is find the difference then find the first 9 terms then add them right?
anonymous
  • anonymous
Find the sum of the first 9 terms of the sequence. Show all work for full credit. 2, -7, -16, -25, ...
anonymous
  • anonymous
for that one they probably want to see you use the formulas. Can you come up with a equation for a_n using the formula?
anonymous
  • anonymous
n-9?
anonymous
  • anonymous
Well ye but if you should use equations same thing so d=+3 and they are 7 terms so n=7 S(7)=7/2 (-10+8)=7/2(2) =7
anonymous
  • anonymous
this one. where you have to find d and a_1 \[a_n=a_1+d(n-1)\]
anonymous
  • anonymous
Sorry -7*
anonymous
  • anonymous
so an=2 + -9(n-1)?
anonymous
  • anonymous
This is a straight arithmetic sequence guys no need for this d is simply d=Un+1 -Un
anonymous
  • anonymous
yes. and they want the sum of the first 9 so now find a_9
anonymous
  • anonymous
a9 would = -70 right?
anonymous
  • anonymous
yes. Now for the sum use this formula \[S_n=\frac{ n }{ 2 }(a_1+a_n)\] so \[S_9=\frac{ n }{ 2 }(a_1+a_9)\]
anonymous
  • anonymous
so the sum would =-306
anonymous
  • anonymous
yes
anonymous
  • anonymous
The sum of the first 9 is 18 S9=n/2 ( 2a1+(n-1)d = 9/2 (-20+24) =36/2 =18
anonymous
  • anonymous
Oh sorry used another sequence
anonymous
  • anonymous
@joyraheb different set of numbers from the sequence at the very top
anonymous
  • anonymous
ok good because i was about to say, i thought i finally understood this then that messed me up hahaha
anonymous
  • anonymous
but thank you!
anonymous
  • anonymous
Oh ok so d=-9 so S9=-4.5(-14+90)= -342 right?
anonymous
  • anonymous
no s9= (9/2)(2+(-70)) = -306
anonymous
  • anonymous
because of this equation Sn=n/2(a1+an) @joyraheb

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