Q6 of 69 Page 78

On the world environment day tree plantation programme was arranged on a land which is triangular in shape. Trees are planted such that in the first row there is one tree, in the second row there are two trees, in the third row three trees and so on. Find the total number of trees in the 25 rows.

First term a = 1

Second term t1 = 2


Third term t3 = 3


Common difference d = t3 – t2 = 3 – 2 = 1


We need to find total number of trees when n = 25


Thus, By using sum of nth term of an A.P. we will find it’s sum



Where, n = no. of terms


a = first term


d = common difference


Sn = sum of n terms


We need to find S25


Thus, on substituting the given value in formula we get,





⇒S25 = 25 × 13 = 325


More from this chapter

All 69 →
4

There is an auditorium with 27 rows of seats. There are 20 seats in the first row, 22 seats in the second row, 24 seats in the third row and so on. Find the number of seats in the 15th row and also find how many total seats are there in the auditorium?

 5

Kargil’s temperature was recorded in a week from Monday to Saturday. All readings were in A.P. The sum of temperatures of Monday and Saturday was 5° C more than sum of temperatures of Tuesday and Saturday. If temperature of Wednesday was – 30° celsius then find the temperature on the other five days.

 1

Choose the correct alternative answer for each of the following sub questions.

The sequence – 10, – 6, – 2, 2, . . .

 1

Choose the correct alternative answer for each of the following sub questions.

First four terms of an A.P. are ....., whose first term is – 2 and common difference is – 2.