Sum

Shubhankar invested in a national savings certificate scheme. In the first year he invested ₹ 500, in the second year ₹ 700, in the third year ₹ 900 and so on. Find the total amount that he invested in 12 years

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#### Solution

Amount invested by Shubhankar in the national savings certificate scheme is as follows:

500, 700, 900, ......

The above sequence is an A.P.

∴ a = 500, d = 700 – 500 = 200, n = 12

Now, S_{n} = `"n"/2 [2"a" + ("n" - 1)"d"]`

∴ S_{12} = `12/2 [2(500) + (12 - 1)(200)]`

= 6[1000 + 11(200)]

= 6(1000 + 2200)

= 6(3200)

= 19200

∴ The total amount invested by Shubhankar is ₹ 19,200.

Concept: Sum of First n Terms of an AP

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