Sequences & Series
Arithmetic Progression
GRB_1000_SCQ
Grade Class 11
Question:
Given $S_A = 2m + (2m+1) + (2m+2) + \ldots + 4m$ and $S_B = (2m+1) + (2m+3) + (2m+5) + \ldots + (4m-1)$. If $\dfrac{S_A}{S_B} = k + \dfrac{1}{l}$, find the value of $k + l$.
Step-by-Step Solution
Key Concept: Sum of Arithmetic Progressions
Step 1: Find the sum $S_A$ by identifying it as an arithmetic progression.
$S_A$ is the sum of consecutive integers from $2m$ to $4m$ with common difference $1$.
The number of terms is:
$$n_A = 4m - 2m + 1 = 2m + 1$$
Using the arithmetic progression sum formula $S = \frac{n(\text{first term} + \text{last term})}{2}$:
$$S_A = \frac{(2m+1)(2m + 4m)}{2} = \frac{(2m+1)(6m)}{2} = 3m(2m+1)$$
Step 2: Find the sum $S_B$ by identifying it as an arithmetic progression of odd numbers.
$S_B$ is the sum of odd numbers from $2m+1$ to $4m-1$ with common difference $2$.
The number of terms is:
$$n_B = \frac{(4m-1)-(2m+1)}{2} + 1 = \frac{2m-2}{2} + 1 = m - 1 + 1 = m$$
Using the arithmetic progression sum formula:
$$S_B = \frac{m(2m+1 + 4m-1)}{2} = \frac{m(6m)}{2} = 3m^2$$
Step 3: Calculate the ratio $\frac{S_A}{S_B}$ and express it in the form $k + \frac{1}{l}$.
$$\frac{S_A}{S_B} = \frac{3m(2m+1)}{3m^2} = \frac{2m+1}{m}$$
Dividing the numerator by the denominator:
$$\frac{2m+1}{m} = \frac{2m + 1}{m} = 2 + \frac{1}{m}$$
Step 4: Identify the values of $k$ and $l$, then find $k + l$.
From the expression $\frac{S_A}{S_B} = 2 + \frac{1}{m}$, we can match it with the form $k + \frac{1}{l}$:
$$k = 2 \quad \text{and} \quad l = m$$
Therefore:
$$k + l = 2 + m$$
The answer is **Option 1: $2 + m$**
Correct Answer: 3