Question

If the set $R = {(a, b) : a + 5b = 42, a, b \in \mathbb{N}}$ has $m$ elements and $\sum_{n=1}^m (1 + i^n) = x + iy$, where $i = \sqrt{-1}$, then the value of $m + x + y$ is:

• A. 8
• B. 12
• C. 4
• D. 5

Answer 1

Answer: (B)

12

Answer 2

From $a + 5b = 42$, where $a, b \in \mathbb{N}$, we have:
$a = 42 - 5b.$
Since $a > 0$, we require:$42 - 5b > 0 \implies b < \frac{42}{5} \implies b \leq 8.$
The possible values of $(a, b)$ are:
$(37, 1), (32, 2), (27, 3), (22, 4), (17, 5), (12, 6), (7, 7), (2, 8).$
Thus, $m = 8$.
The sum is:
$\sum_{n=1}^{8}(1 - i^n).$
For $n \geq 4$, $i^n$ repeats every 4 terms:
$i^1 = i, \; i^2 = -1, \; i^3 = -i, \; i^4 = 1.$
Compute:
$\sum_{n=1}^{8}(1 - i^n) = (1 - i) + (1 - (-1)) + (1 - (-i)) + (1 - 1) + (1 - i) + (1 - (-1)) + (1 - (-i)) + (1 - 1).$
Simplify:
$= (1 - i) + 2 + (1 + i) + 0 + (1 - i) + 2 + (1 + i) + 0 = 5 - i + i = 5.$
Thus:
$x + y = 5, \; m = 8, \; m + x + y = 8 + 5 - 1 = 12.$
Final Answer: 12.