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 one tree, a section of class XII will plant two trees and so on till class XII. There are three sections of each class. How many trees will be planted by the students?

Answer 1

Hyy!!
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Number of classes = 12
∵ Each class has 3 sections.

∴ Number of plants planted by class 1 = 1*3 = 3
   Number of plants planted by class 2 = 2*3 = 6
   Number of plants planted by class 3 = 3*3 = 9
   Number of plants planted by class 4 = 4*3 = 12
    .............................................................................

   Number of plants planted by class 12 = 12*3 = 36
   The numbers 3,6,9,12,....36 are in A.P

Here, a=3 , d= 6-3 = 3 and l = an= 36

Let the last last term of this A.P is its nth term, then

∵ an = L = a+(n-1)d
∴ 3 + (n-1) (3) = 36
   3(n-1) = 36 - 3
   n - 1 = 33/3 = 11
   n = 11 +1 = 12 
 
 Hence, the number of trees planted by the students = sum of 12 terms of above A.P.
 
= 12/2 (3+36)                      {∵ Sn = n/2 (a+l) }
= 6*39
= 234

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Hope it helps you!!
                                             

Answer 2

Answer:

your answer is in the pic