<p>If L is the line whose equation is \(ax + by = c\). Let M be the reflection of L through the y-axis, and let N be the reflection of L through the x-axis. Which of the following must be true about M and N for all choices of a, b and c?</p>
<p>(a) The x-intercepts of M and N are equal</p>
<p>(b) The y-intercepts of M and N are equal</p>
<p>(c) The slopes of M and N are equal</p>
<p>(d) The slopes of M and N are reciprocal</p>
Step-by-Step Solution
Key Concept: Understand how reflections through coordinate axes transform line equations and affect intercepts
Let the equation of line L be \(ax + by = c\).
The slope of line L is given by \(m_L = -\frac{a}{b}\), provided \(b \neq 0\). If \(b=0\), L is a vertical line with an undefined slope.
**1. Determine the equation and slope of line M.**
Line M is the reflection of L through the y-axis. To find the equation of M, replace \(x\) with \(-x\) in the equation of L:
$$a(-x) + by = c$$
$$-ax + by = c$$
To find the slope of M, rearrange the equation into slope-intercept form \(y = mx + k\):
$$by = ax + c$$
$$y = \frac{a}{b}x + \frac{c}{b}$$
The slope of M is \(m_M = \frac{a}{b}\), provided \(b \neq 0\).
**2. Determine the equation and slope of line N.**
Line N is the reflection of L through the x-axis. To find the equation of N, replace \(y\) with \(-y\) in the equation of L:
$$ax + b(-y) = c$$
$$ax - by = c$$
To find the slope of N, rearrange the equation into slope-intercept form \(y = mx + k\):
$$-by = -ax + c$$
$$y = \frac{a}{b}x - \frac{c}{b}$$
The slope of N is \(m_N = \frac{a}{b}\), provided \(b \neq 0\).
**3. Compare the slopes of M and N.**
From the derivations, the slope of M is \(m_M = \frac{a}{b}\) and the slope of N is \(m_N = \frac{a}{b}\).
Thus, \(m_M = m_N\).
This holds true when \(b \neq 0\). If \(b=0\), line L is \(ax=c\), which is a vertical line.
In this case:
Line M: \(-ax=c\), which is also a vertical line.
Line N: \(ax=c\), which is also a vertical line.
Vertical lines have undefined slopes, but they are parallel, meaning their slopes are considered equal in a geometric context.
Therefore, the slopes of M and N are equal for all choices of a, b, and c.
Correct Answer: b