Question 66

The sum of first 9 terms of the series $$\frac{1^3}{1} + \frac{1^3 + 2^3}{1+3} + \frac{1^3 + 2^3 + 3^3}{1+3+5} + \ldots$$ is

We begin by observing the pattern of the given series

$$\frac{1^3}{1}\;+\;\frac{1^3+2^3}{1+3}\;+\;\frac{1^3+2^3+3^3}{1+3+5}\;+\;\ldots$$

For the general $$n^{\text{th}}$$ term, the numerator contains the sum of the first $$n$$ cubes and the denominator contains the sum of the first $$n$$ odd numbers. Let us treat each of these sums separately.

Numerator: The sum of cubes formula states

$$1^3+2^3+\ldots+n^3=\left[\frac{n(n+1)}{2}\right]^2.$$

Denominator: The sum of the first $$n$$ odd numbers is a well-known result

$$1+3+5+\ldots+(2n-1)=n^2.$$

Hence the $$n^{\text{th}}$$ term $$T_n$$ of the series is

$$T_n=\frac{\displaystyle\left[\dfrac{n(n+1)}{2}\right]^2}{n^2}.$$

We now simplify $$T_n$$. First we expand the square in the numerator:

$$\left[\frac{n(n+1)}{2}\right]^2=\frac{n^2(n+1)^2}{4}.$$

Dividing this by the denominator $$n^2$$ gives

$$T_n=\frac{\,\dfrac{n^2(n+1)^2}{4}\,}{n^2} =\frac{(n+1)^2}{4}.$$

Therefore each term of the series is simply a quarter of a perfect square:

$$T_n=\frac{(n+1)^2}{4}.$$

We are asked for the sum of the first 9 terms, so we must evaluate

$$S_9=\sum_{n=1}^{9}T_n=\sum_{n=1}^{9}\frac{(n+1)^2}{4}.$$

Since the factor $$\frac14$$ is common to every term, we can factor it outside the summation:

$$S_9=\frac14\sum_{n=1}^{9}(n+1)^2.$$

To re-index the summation more comfortably, let us put $$m=n+1$$. Then, when $$n=1$$ we have $$m=2$$ and when $$n=9$$ we have $$m=10$$. Hence

$$\sum_{n=1}^{9}(n+1)^2=\sum_{m=2}^{10}m^2.$$

We now use the formula for the sum of the squares of the first $$k$$ natural numbers:

$$1^2+2^2+\dots+k^2=\frac{k(k+1)(2k+1)}{6}.$$

First we compute the sum up to $$k=10$$:

$$\sum_{m=1}^{10}m^2=\frac{10\,(10+1)\,(2\cdot10+1)}{6} =\frac{10\cdot11\cdot21}{6} =\frac{2310}{6} =385.$$

But we need the sum from $$m=2$$ to $$m=10$$, so we subtract the first term $$1^2=1$$:

$$\sum_{m=2}^{10}m^2=385-1=384.$$

Substituting this result back into our expression for $$S_9$$, we get

$$S_9=\frac14\times384=96.$$

Thus the total of the first nine terms of the given series equals 96.

Hence, the correct answer is Option C.

Was this solution helpful?

Get AI Help

Video Solution

video

Create a FREE account and get:

  • Free JEE Mains Previous Papers PDF
  • Take JEE Mains paper tests
Ask AI