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प्रश्न
Sachin invested some amounts in National Saving Certificates in a specific way. In the first year, he invested 4,000 in the second year 6,000 in the third year 8,000 and so on for 12 years. Find the total amount he invested in 12 years.
उत्तर
Sachin invested in first year = 4000
Second year investment = 6000
Third year investment = 8000
Total investment = Rs (4000 + 6000 + 8000 + .... + in 12 years)
Here,
a = 4000
d = 2000
n = 12
Now ,
`"S"_"n" = "n"/2 [2"a" + ("n"-1)"d"]`
`"S"_12 = 12/2 [2(4000) + (12 - 1)(2000)]`
`= 6 [8000 + 22000]`
= 6 × 30000
= Rs 180000
Hence, the total amount that he invested in 12 years is Rs 180000.
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