In a school, students thought of planting trees in and around the school to reduce air pollution. It was decided that the number of trees, that each section of each class will plant, will be the same as the class, in which they are studying, e.g., a section of Class I will plant 1 tree, a section of Class II will plant 2 trees and so on till Class XII. There are three sections of each class. How many trees will be planted by the students?

Given that each section of each class will plant a trees.

e.g., a section of Class I will plant 1 tree, a section of Class II will plant 2 trees and so on till Class XII. There are three sections.

So the trees planted by each class will be like this

1 × 3, 2 × 3, 3 × 3,…. 12 × 3

⇒ 3, 6, 9,…..36

In the above series

First term (a) = 3

Common difference (d) = a₂ – a₁ = 6-3 = 3

The sum of 12 terms is given by (s_n) = [2a + (n-1) d]

S₁₂ = [2×3 + (12-1) 3]

= 6( 6+33) = 234

So, totally 234 trees will be planted by students in the school.