Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
The sum of the series $$1 + 2 \times 3 + 3 \times 5 + 4 \times 7 + \ldots$$ upto 11th term is:
First we look at the terms one by one. We have $$1 ,\; 2 \times 3 ,\; 3 \times 5 ,\; 4 \times 7 ,\; \ldots$$ and so on. Observing the pattern, the first factor of the $$n^{\text{th}}$$ term is simply $$n$$ while the second factor is always one less than twice that $$n$$, namely $$(2n-1).$$
So the general $$n^{\text{th}}$$ term can be written as
$$T_n \;=\; n \,(2n-1).$$
Now we expand this product to make later addition easier:
$$T_n \;=\; n\,(2n-1) \;=\; 2n^2 - n.$$
We are asked to find the sum of the first eleven terms, so we must add $$T_1 + T_2 + \ldots + T_{11}.$$ Symbolically, that sum is
$$S_{11} \;=\; \sum_{n=1}^{11} \bigl(2n^2 - n\bigr).$$
Because summation distributes over addition (that is, $$\sum (A+B)=\sum A+\sum B$$), we separate the two parts:
$$S_{11} \;=\; 2 \sum_{n=1}^{11} n^2 \;-\; \sum_{n=1}^{11} n.$$
At this point we use two well-known formulae for consecutive integers.
First formula (sum of the first $$m$$ natural numbers):
$$\sum_{n=1}^{m} n \;=\; \frac{m(m+1)}{2}.$$
Second formula (sum of the squares of the first $$m$$ natural numbers):
$$\sum_{n=1}^{m} n^2 \;=\; \frac{m(m+1)(2m+1)}{6}.$$
Here, $$m = 11.$$ Substituting $$m=11$$ into the second formula gives
$$\sum_{n=1}^{11} n^2 \;=\; \frac{11 \times 12 \times 23}{6}.$$
We multiply step by step:
$$11 \times 12 = 132,$$
$$132 \times 23 = 3036,$$
$$\frac{3036}{6} = 506.$$
So we have
$$\sum_{n=1}^{11} n^2 = 506.$$
Next, substituting $$m=11$$ into the first formula gives
$$\sum_{n=1}^{11} n \;=\; \frac{11 \times 12}{2} \;=\; \frac{132}{2} \;=\; 66.$$
Now we return to our expression for $$S_{11}$$:
$$S_{11} \;=\; 2 \times 506 \;-\; 66.$$
First carry out the multiplication:
$$2 \times 506 = 1012.$$
Then perform the subtraction:
$$1012 - 66 = 946.$$
Thus the required sum of the first eleven terms is
$$S_{11} = 946.$$
Hence, the correct answer is Option C.
Click on the Email ☝️ to Watch the Video Solution
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.