Question

The sum of the first 8 terms of an arithmetic sequence is -20. Find the first 4 terms of the sequence if the 8th term is 8

Answer 1

an = a1 + (n – 1)d
an is the nth term
d is the difference between successive terms
a1 is the first term

the sum is (a1+an)/2

Answer 2

so for you, an =8 a1 = ? sum =-20
find a1
use it to find d in
an = a1 + (n – 1)d

Answer 3

from there it's easy to find the other terms

Answer 4

oh ok

Answer 5

what did you get for a1?

Answer 6

is it -4?

Answer 7

-20 = (8+a1)/2

Answer 8

multiply both sides by 2
subtract 8 from both sides...
a1 =?

Answer 9

whoops left out an n my bad

Answer 10

-20=n*(8+a1)/2

so -20 = 8(8+a1)/2

-20/4 = 8+a1
a1=-13

Answer 11

sorry my algebra skills a little rusty

Answer 12

mine too evidently. :)

Answer 13

are you happy with our a1? or do you have questions?

Answer 14

oh yeah i understand how u got al

Answer 15

cool.

Answer 16

show me how to find 'd' now... suing the value for a1 we just found.

Answer 17

using the value*

Answer 18

i got 3

Answer 19

good.

Answer 20

so first term is -13 second term is -13 +3 third term is -13 +6 and so on...

Answer 21

so answers are -13, -10, -7 and -4

Answer 22

is that right?

Answer 23

yep:)

Answer 24

kk cool thanks

Answer 25

sure!