Question

If Sn=25 and an=10 then Sn-1= ?

Answer 1

Answer:

The correct answer for the sum of n-1 terms, i.e., $S_{n-1}$ is found to be 15.

Step-by-step explanation:

We are given the sum of n terms of an Arithmetic progression series as:

$S_n=25$

Similarly the nth term of an AP is defined as: $a_n=a+(n-1)d$ which is given to us as 10, so:

$a_n=10$

We know that to find the sum of n-1 terms of an AP using the sum of n terms, we need to subtract the nth term from the total sum (upto n terms).

Thus, we can represent this as:

$S_{n-1}=S_n-a_n$

Substituting the given values, we get:

$S_{n-1}=25-10\\S_{n-1}=15$

Thus, the required answer comes out to be 15.

Answer 2

Answer:

The correct answer is 15

Step-by-step explanation:

Given, Sn=25 & an=10

Find, Sn-1

We know that, an=a + (n-1)d

where an= the nth term of an AP

Sn= sum of nth term of an AP

therefore,

Sn-1 =Sn - an

After putting the values,

Sn-1= 25-10=15

So, the final answer is 15 after applying formula and putting values of Sn and an