The equation of the line of symmetry

To describe a reflection on a grid, the equation of the mirror line is needed.

Example

Reflect the shape in the line .

The line is a vertical line which passes through -1 on the -axis.

Question

Describe the transformation of the shape ABC.

The line is a horizontal line which passes through 1 on the -axis.

The shape has been reflected in the line .

Question

Reflect the shape in the line .

The equation of a straight line graph has the form , where is the gradient and is where the line crosses the -axis.

The line has a gradient of 1 and crosses the -axis at (0,0).

Question

Describe the transformation of the shape ABC.

The mirror line has a gradient of -1 and crosses the -axis at (0,0).

The shape has been reflected in the line .

The equation of the mirror line is needed to fully describe a reflection.