Question

What is the 31st term of this sequence?


100, 91, 82, 73, 64, ...

Answer 1

I need the formula

Answer 2

the difference is:
\(\large\color{black}{ (a_{n})-(a_{n-1}) }\)
it can be found for example using,
\(\large\color{black}{ (a_{2})-(a_{1}) }\)

Answer 3

okay thank you ! (:

Answer 4

now we know that: \(\large\color{black}{ (a_{n})=(a_1)+d(n-1) }\)

So for 31st term, we get: \(\large\color{black}{ (a_{31})=(a_1)+d(30) }\)

Answer 5

is it -4185?

Answer 6

And \(a_1 = 100\)

Answer 7

it is not -4185.

Answer 8

let me try again

Answer 9

sure;)

Answer 10

-1085

Answer 11

\(\large\color{black}{ (a_{31})=(a_{1})-30(d) }\)

Answer 12

what have you found the difference (d) to equal to?

Answer 13

-170

Answer 14

but its it a + instead of - ?

Answer 15

Yes, -170 is right:)

Answer 16

can you tell me what i did wrong?

Answer 17

I don't know what exactly you did, because you haven't posted any work, have you?

Answer 18

okay well first i did the whole a1+d(n-1) i got -170

after i put it together 31(100+ (-170) / 2 and i got -1085

Answer 19

so you did it twice pretty much?
you first did,
\(\large\color{black}{ (a_{31})=(a_1)-d(30) }\)
\(\large\color{black}{ (a_{31})=100-9(30) }\)
\(\large\color{black}{ (a_{31})=100-270 }\)
\(\large\color{black}{ (a_{31})=-170 }\)

Answer 20

and then you plugged -170 for difference again, to re-find the \(\large\color{black}{ (a_{31}) }\) ?

Answer 21

no no cause i have a different thing that n(a1+an)/2

Answer 22

well, now at least you know what to do to get the \(\normalsize\color{blue}{ \rm correct }\) answer?