How do you find the number of terms in an arithmetic sequence when only knowing the sum?

Question

parthkohli · Answer

The formula for sum is:

$\Large \color{MidnightBlue}{\Rightarrow {n \over 2}(a_1 + a_n) }$

ajprincess · Answer

This is alternative one to find the sum.
Sn=n/2 [2a + (n - 1)d]

lgbasallote · Answer

$$\large S_n = \frac{n(a_n + a_1)}{2}$$ $$\large n = \frac {2S_n}{a_n+a_1}$$

parthkohli · Answer

Duh!

lgbasallote · Answer

why didnt you think of that then @ParthKohli

parthkohli · Answer

Lol I was making him work it out

anonymous · Answer

wait i'm interested in learning this ... could you please put in what each letter/variable represents.

cwtan · Answer

Sn= Sum of nth term
a n= nth term

anonymous · Answer

so whats a represent?

cwtan · Answer

If the series
1,2,3,4,5
a1= First Term =1
a2=Second Term=2
an=nth Term
If n =5 then an=5

ajprincess · Answer

d-common difference

cwtan · Answer

Oops I 4gt abt it

ajprincess · Answer

no problem.

lgbasallote · Answer

A1 means first term
An means last term @aceace

anonymous · Answer

thanks... lol i am still to learn this stuff. is this topic in maths that difficult?

cwtan · Answer

Different for every person... But for me it's quite easy

ajprincess · Answer

anonymous · Answer

thank you guys ;)