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.

According to the question 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 n^th term of an A.P. we will find its sum

Where, n = no. of terms
a = first term
d = common difference
S_n = 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