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?

Each section of each class will plant tree = 3 × Class, therefore
Total number of tree planted by class i = 3 × 1 = 3
Total number of tree planted by class ii = 3 × 2 = 6
Total number of tree planted by class iii = 3 × 3 = 9
Similary, the series of trees planted by classes are as follows: 3, 6, 9, …., 36
Here, a = 3, d = 6 – 3 = 3 and n = 12.
The sum of n terms of an AP is given by S_n = n/2[2a + (n -1)d]
⇒ S₁₂ = 12 /2[2(3) + (12 – 1)(3)]
= 6[6 + 33] = 6(39) = 234
Hence, the total number of tree planted by the students is 234.