(Q34).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: Classes: From I to XII

Section: 3 in each class.

∴ Trees planted by each class = 3 × class number

Class I II III IV V VI VII VIII IX X XI XII
Trees planted 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × = 3 × =

∴ Total trees planted = 3 + 6 + 9 + 12 + …… + 36 is an A.P.

Here, a = and l = ; n =

∴ Sn =
n
2
 (a+l)
S12 =
2
 [ + ]

= ×

=

∴ Total plants =