Answer:
Step-by-step explanation:
It's very necessary to find whether the given sequence is arithmetic or geometric.
In Arithmetic sequence every successive term has a common difference and in geometric sequence every successive term shows a common ratio.
Example of Arithmetic sequence:
1, 2, 3, 4, 5......
2 - 1 = 1
3 - 2 = 1
common difference of 1.
Example of geometric sequence:
2, 4, 8, 16..........


Common ratio of 2
Now formula to calculate the sum of initial n terms of Arithmetic sequence
![S_{n}=\frac{n}{2}[2a+(n-1)d]](https://tex.z-dn.net/?f=S_%7Bn%7D%3D%5Cfrac%7Bn%7D%7B2%7D%5B2a%2B%28n-1%29d%5D)
where a = first term
n = number of terms to be added
d = common difference
Formula to calculate the sum of initial n terms of the Geometric sequence

where a = first term of the sequence
n = number of terms to be added
r = common ratio
With help of these formula we can find the sum of first 25 terms of any sequence given.