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Question:

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?

Solution:

It can be observed that the number of trees planted by the students is in an AP.1,2,3,4,5………………..12First term,a=1Common difference,d=2−1;=1Sn=n2[2a+(n−1;)d]S12=122[2(1)+(12−1;)(1)]=6(2+11)=6(13)=78Therefore, number of trees planted by1section of the classes=78Number of trees planted by3sections of the classes=3×78=234Therefore,234trees will be planted by the students.