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# Question 61. In a school, students decided to plant 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 double of the class in which they are studying. If there are 1 to 12 classes in the school and each class has two sections, find how many trees were planted by the students.

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One of the most important and exam oriented question from Chapter name- Arithmetic Progression
Class 10th
Chapter number- 9
Exercise :- 9.6
This type of question has been asked in previous years exams.

In this question we have been given that In a school, students decided to plant 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 double of the class in which they are studying.

And there are 1 to 12 classes in the school and each class has two sections,

Now we have to find how many trees were planted by the students.

CBSE DHANPAT RAI publication
CBSE Mathematics Class 10th
Question 61

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1. Trees planted by the student in a class A = 2(A) [Since there are 2 sections for every class.]

Trees planted by the students from class 1 to 12 in sequence can be written as:

4, 8, 12,………., 48

First term(a) = 4 and common difference, d = 8 − 4 = 4

By using the formula of nth term of an A.P.

an = a + (n – 1)d

So,

=> 48 = 4 + (n − 1)4

=> 4(n − 1) = 44

=> n − 1 = 11

=> n = 12

Now by using the formula of sum of n terms of an A.P.

Sn = n[a + an] / 2

According to the question,

Number of trees planted = Sum of the A.P. series = S12

= 12[4+48]/2 = 6 = 312

Hence, 312 trees were planted by the students.

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