Which point would map onto itself after a reflection across the line [tex]$y = -x$[/tex]?

A. [tex][tex]$(-4, -4)$[/tex][/tex]
B. [tex]$(-4, 0)$[/tex]
C. [tex]$(0, -4)$[/tex]
D. [tex][tex]$(4, -4)$[/tex][/tex]



Answer :

To determine which point maps onto itself after a reflection across the line [tex]\( y = -x \)[/tex], we need to analyze what happens to a point [tex]\((a, b)\)[/tex] when it is reflected over this line.

The formula for reflecting a point [tex]\((a, b)\)[/tex] across the line [tex]\( y = -x \)[/tex] results in the new coordinates [tex]\((-b, -a)\)[/tex].

For a point to map onto itself, its coordinates must satisfy the condition:
[tex]\[ (a, b) = (-b, -a) \][/tex]
This leads to the following system of equations:
[tex]\[ a = -b \][/tex]
[tex]\[ b = -a \][/tex]

These are two expressions of the same condition, essentially meaning [tex]\( a \)[/tex] must be the negative of [tex]\( b \)[/tex].

Let's now analyze each point given in the problem:

1. Point (-4, -4):
- Reflect [tex]\((-4, -4)\)[/tex] across [tex]\( y = -x \)[/tex]:
[tex]\[ (-(-4), -(-4)) = (4, 4) \][/tex]
[tex]\((-4, -4)\)[/tex] does not map onto itself.

2. Point (-4, 0):
- Reflect [tex]\((-4, 0)\)[/tex] across [tex]\( y = -x \)[/tex]:
[tex]\[ (0, 4) \][/tex]
[tex]\((-4, 0)\)[/tex] does not map onto itself.

3. Point (0, -4):
- Reflect [tex]\((0, -4)\)[/tex] across [tex]\( y = -x \)[/tex]:
[tex]\[ (4, 0) \][/tex]
[tex]\((0, -4)\)[/tex] does not map onto itself.

4. Point (4, -4):
- Reflect [tex]\( (4, -4)\)[/tex] across [tex]\( y = -x \)[/tex]:
[tex]\[ (4, -4) \rightarrow (4, -4) \][/tex]

We see that [tex]\( (4, -4) \)[/tex] maps back to itself.

By analyzing each point, we conclude that the point that maps onto itself after a reflection across the line [tex]\( y = -x \)[/tex] is:

[tex]\( \boxed{(4, -4)} \)[/tex]