Reflecting B(3,-1) on Line g: 4y + x - 15 = 0

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In summary, the point B(3, -1) reflects to B'(5, 7) when passing through a line g with the equation 4y + x - 16 = 0. This is determined by finding the midpoint of the given points and using the perpendicular slope to form the equation of the line through the midpoint. Option A is likely a typo.
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Monoxdifly
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The point B(3, -1) is reflected by the line g and results in B'(5, 7). The equation of line g is ...
A. 4y + x - 15 = 0
B. 4y + x - 9 = 0
C. 4y + x + 15 = 0
D. 4y - x - 15 = 0
E. 4y - x - 9 = 0

Since I didn't know how to approach the problem in a formal, textbook way, I tried to get... creative. The point of reflection must be exactly in the middle of (3, -1) and (5, 7), that is, (4, 3). Since the mirror must be a line perpendicular to BB' (which has the slope 4) and going through (4, 3), the slope of the mirror is \(\displaystyle -\frac{1}{4}\) and I substituted it in the \(\displaystyle y-y_1=m(x-x_1)\) equation. This is what I got:
\(\displaystyle y-3=-\frac{1}{4}(x-4)\)
4(y - 3) = -(x - 4)
4y - 12 = -x + 4
4y + x - 12 - 4 = 0
4y + x - 16 = 0 which is not in any of the options, but really close to the option A. Can we just assume that the option A was a typo? Or did I make a mistake somewhere?
 
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  • #2
What I would do is observe that the line must pass through the midpoint of the two given points, and be perpendicular to the line through the two given points.The midpoint is:

\(\displaystyle \left(\frac{3+5}{2},\frac{-1+7}{2}\right)=(4,3)\)

The slope is:

\(\displaystyle m=-\frac{\Delta x}{\Delta y}=-\frac{5-3}{7+1}=-\frac{1}{4}\)

Thus, our line is:

\(\displaystyle y-3=-\frac{1}{4}(x-4)\)

Or:

\(\displaystyle 4y-12=-x+4\)

Or:

\(\displaystyle 4y+x-16=0\)

I agree with your answer. :)
 
  • #3
Isn't that basically what I did?
 
  • #4
Monoxdifly said:
Isn't that basically what I did?

Yes...it's just easier for me to work the problem and then compare results.
 

FAQ: Reflecting B(3,-1) on Line g: 4y + x - 15 = 0

What is the equation of the line g?

The equation of line g is 4y + x - 15 = 0.

What is the point being reflected, B(3,-1)?

The point being reflected is B(3,-1), which has coordinates of x=3 and y=-1.

What is the formula for reflecting a point over a line?

The formula for reflecting a point (x,y) over a line ax + by + c = 0 is (x',y') = ((x - 2ax - 2by + 2c)/(a^2 + b^2), (y - 2bx + 2ay + 2c)/(a^2 + b^2)).

What are the steps for reflecting a point over a line?

The steps for reflecting a point (x,y) over a line ax + by + c = 0 are:
1. Find the slope of the line, m = -a/b
2. Find the equation of the perpendicular line, y = mx + d
3. Find the coordinates of the reflected point using the formula (x',y') = ((x - 2mx + 2y - 2d)/(m^2 + 1), (mx + x + 2my + 2d)/(m^2 + 1))
4. Replace the original coordinates with the reflected coordinates to get the reflected point.

How do I know if the point is above or below the line after reflecting?

If the point is above the line, the y-coordinate of the reflected point will be positive. If the point is below the line, the y-coordinate of the reflected point will be negative.

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