Question

Find the sum of the arithmetic sequence.

-10, -7, -4, -1, 2, 5, 8

11
-70
0
-7

Answer 1

@misssunshinexxoxo

Answer 2

@peachpi

Answer 3

Goshhh cant you add them just add them you get -7

Answer 4

oh so all i do is add?

Answer 5

that's what I'd do since they're listed

Answer 6

what if they give me only 4 terms and tell me to find the sum of the first 9

Answer 7

nevermind all you do is find the difference then find the first 9 terms then add them right?

Answer 8

Find the sum of the first 9 terms of the sequence. Show all work for full credit.

2, -7, -16, -25, ...

Answer 9

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?

Answer 10

n-9?

Answer 11

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

Answer 12

this one. where you have to find d and a_1
\[a_n=a_1+d(n-1)\]

Answer 13

Sorry -7*

Answer 14

so an=2 + -9(n-1)?

Answer 15

This is a straight arithmetic sequence guys no need for this d is simply d=Un+1 -Un

Answer 16

yes. and they want the sum of the first 9 so now find a_9

Answer 17

a9 would = -70 right?

Answer 18

yes. Now for the sum use this formula
\[S_n=\frac{ n }{ 2 }(a_1+a_n)\]
so
\[S_9=\frac{ n }{ 2 }(a_1+a_9)\]

Answer 19

so the sum would =-306

Answer 20

yes

Answer 21

The sum of the first 9 is 18 S9=n/2 ( 2a1+(n-1)d = 9/2 (-20+24) =36/2 =18

Answer 22

Oh sorry used another sequence

Answer 23

@joyraheb different set of numbers from the sequence at the very top

Answer 24

ok good because i was about to say, i thought i finally understood this then that messed me up hahaha

Answer 25

but thank you!

Answer 26

Oh ok so d=-9 so S9=-4.5(-14+90)= -342 right?

Answer 27

no s9= (9/2)(2+(-70)) = -306

Answer 28

because of this equation Sn=n/2(a1+an) @joyraheb