Q3 of 69 Page 78

Sachin invested ina national saving certificate scheme. In the first year he invested ₹ 5000, in the second year ₹ 7000, in the third year ₹ 9000 and so on. Find the total amount that he invested in 12 years.

By given information we can form an A.P.

5000, 7000, 9000, ……


Hence, the first term a = 5000


Second term t1 = 7000


Third term t2 = 9000


Thus, common difference d = t2 – t1 = 9000 – 7000 = 2000


Here, number of terms n = 12


We need to find S12


Now, 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


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



⇒S12 = 6 × [ 10,000 + 11 × 2000]


⇒S12 = 6 × [10,000 + 22,000]


⇒S12 = 6 × 32,000


⇒S12 = Rs. 192000


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