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

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,â€¦â€¦â€¦., 48

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

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

an = a + (n â€“ 1)d

So,

=> 48 = 4 + (n âˆ’ 1)4

=> 4(n âˆ’ 1) = 44

=> n âˆ’ 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[52] = 312

Hence, 312 trees were planted by the students.