Question

...

Answer 1

hi :)
do you know the formula for n'th term of arithmetic series ?

Answer 2

Nope hehe

Answer 3

hehe, so here it is!
\(\large a_n=a_1+(n-1)d\)
where a1 is the 1st term and d is the common difference :)

Answer 4

Thanks! :D

Answer 5

But wait, how can I get the firt term then? :))

Answer 6

yo get 5th term , you put n=5!

Answer 7

can you show the step by step solution? :) :D

Answer 8

ok, lets start.
for 5th term , n=5
so, from that formula
\(\large a_5 = a_1+4d\)
got this ?

Answer 9

yes

Answer 10

you have 8th and 15th terms put n=8 and n =15 you will get two equations solve them
for 8th term
n=8
9= a1+7d
30=a1+14d

Answer 11

oh, wait, why did you need 5th term ?

Answer 12

I mean the first term

Answer 13

Because you have two unknowns... the first term and the common difference ^_^

Answer 14

Oh wait...

Answer 15

lol, ok...the first term is a1, which you need to find.
from 15th and 8th term

Answer 16

Yes...

Answer 17

so, lets start with 8th term
8th term , n=8
so, from that formula
a8=a1+7d
got this ?

Answer 18

yes

Answer 19

15th term , n=15
so, from that formula
a15=a1+14d
got this ?

Answer 20

where did 14 came? hehe

Answer 21

an = a1 + "(n-1)" d

n was 15 , so n-1 = 14

Answer 22

okok :)

Answer 23

Hey, @hartnn if it helps, maybe you can have this instead..
\[\large a_n = a_m +(n-m)d\]
you can find d immediately

Answer 24

we can use that, but since this is just start of the topic, lets keep it simple and standard.

Answer 25

I guess

Answer 26

so, we have
a15=a1+14d = 30
and
a8=a1+7d = 9
can you think of a way to find a1 from here ?
@gcgmpng ?

Answer 27

hint : multiply 2nd equation by 2 (or -2)

Answer 28

what to do after multiplying?

Answer 29

did you multiply ? what you got ?
then you subtract (or add) the 2 equations.

Answer 30

2a8=a1+7d=18?

Answer 31

a8 ??
the 2 equations were
a1+14d = 30
and
a1+7d = 9

Answer 32

after multiplying by 2, in the 2nd equation, you get
2a1 +14 d = 18
right ?

Answer 33

yes

Answer 34

now subtract
a1+14d = 30
from
2a1 +14 d = 18
what you get ?

Answer 35

12

Answer 36

correct!
a1 = 12
good :)
thats your 1st term.
any doubts ?

Answer 37

None :D THANK YOU! \:D/

Answer 38

welcome ^_^

Answer 39

oh, wait!

Answer 40

isn't it a1 = -12 ??

Answer 41

Yes, yes it is!Haha

Answer 42

ok.....glad we corrected that immediately...

Answer 43

Thank you again! :D

Answer 44

then welcome again!