Question

3+5+7+.....+k=440
how i find k?

Answer 1

is about aritmetic?

Answer 2

oops sorry

Answer 3

we have an arithmetic Sequence with d = 2 , a1=3
the sum of n terms is :
(n/2) * (2*a1 + (n-1)*d) = 440

Answer 4

now when we will have 'n' we can find k since it's the nth element
an = a1 + (n-1)*d = k

Answer 5

are you there ?

Answer 6

yes iam...

Answer 7

so are you trying to solve it using what i said ?

Answer 8

now...i wil try to find

Answer 9

ok

Answer 10

where is n?

Answer 11

you have to find it.. using (n/2) * (2*a1 + (n-1)*d) = 440

Answer 12

n is the index of the number k you are looking for

Answer 13

still understand with n

Answer 14

what ?

Answer 15

in my case ,value n is ?

Answer 16

n is the index of the number k ..
a1 = 3 , a2 = 5 ,... , an=k

Answer 17

weneed to find n using :
(n/2) * (2*a1 + (n-1)*d) = 440
then .. when we have n we can find k using :
k = an = a1 + (n-1)*d

Answer 18

ok still try

Answer 19

2n+12(n-1)=440..then?

Answer 20

n=328/12

Answer 21

this is not correct..
(n/2) * (2*a1 + (n-1)*d) = 440
(n/2) * (2*3 + (n-1)*2) = 440
(n/2) * (4 + 2n) = 440
n * (2+ n) = 440
n^2 + 2n = 440

Answer 22

this will give you two solution which one of them will be negative and thus not the right one
since n is index it is positive

Answer 23

so i choose positif index?

Answer 24

yes.. what is n from this equation ?

Answer 25

then enter to an=al-(n-1)d=440 ?

Answer 26

if we found n=20...then a(20)=2-(20-1)3====)an=-55?

Answer 27

an = a1 + (n-1)*d = k

Answer 28

3 + (20 -1 )*2 = 41

Answer 29

there is a plus not a minus between a1 and (n-1)d

Answer 30

you should review the basics of arithmetic sequences ..

Answer 31

ok .thanks brother....

Answer 32

yw:)