On the world environment day tree plantation programme was arranged on a land which is triangular in shape. Trees are planted such that in the first row there is one tree, in the second row there are two trees, in the third row three trees and so on. Find the total number of trees in the 25 rows.

First term a = 1
Second term t₁ = 2
Third term t₃ = 3
Common difference d = t₃ – t₂ = 3 – 2 = 1
We need to find total number of trees when n = 25
Thus, by using sum of n^th term of an A.P. we will find its sum

Where, n = no. of terms
a = first term
d = common difference
S_n = sum of n terms
We need to find S₂₅
Thus, on substituting the given value in formula we get,