Coordinate Moves

Student Summary

We can use coordinates to describe points and find patterns in the coordinates of transformed points.

We can describe a translation by expressing it as a sequence of horizontal and vertical translations.

For example, segment ABAB is translated right 3 and down 2.

Quadrilateral on a coordinate plane.
Quadrilateral on a coordinate plane. Horizontal axis scale negative 3 to 5 by 1’s. Vertical axis scale negative 2 to 3 by 1’s. Quadrilateral A B B prime A prime has coordinates A(negative 2 comma 1), B(1 comma 2), B prime(4 comma 0) and A prime (1 comma negative 1).

Reflecting a point across an axis changes the sign of one coordinate.

For example, reflecting the point AA whose coordinates are (2,-1)(2,\text-1) across the xx-axis changes the sign of the yy-coordinate, making its image the point AA’ whose coordinates are (2,1)(2,1). Reflecting the point AA across the yy-axis changes the sign of the xx-coordinate, making the image the point A’’A’’ whose coordinates are (-2,-1)(\text-2,\text-1).

3 points on a coordinate plane.
3 points on a coordinate plane. Horizontal axis scale negative 3 to 5 by 1’s. Vertical axis scale negative 2 to 2 by 1’s. The points have these coordinates: A(2 comma negative 1), A prime(2 comma 1) and A double prime (negative 2 comma negative 1).

Reflections across other lines are more complex to describe.

We don’t have the tools yet to describe rotations in terms of coordinates in general. Here is an example of a 9090^\circ rotation with center (0,0)(0,0) in a counterclockwise direction.

Point AA has coordinates (0,0)(0,0). Segment ABAB is rotated 9090^\circ counterclockwise around AA. Point BB with coordinates (2,3)(2,3) rotates to point BB’ whose coordinates are (-3,2)(\text-3,2).

Segment A B rotated on a coordinate plane, origin O.
Segment A B rotated on a coordinate plane, origin O. Horizontal axis scale negative 4 to 4 by 1’s. Vertical axis scale negative 2 to 4 by 1’s. The segments have these coordinates: A(0 comma 0), B(2 comma 3) and B prime (negative 3 comma 2). Angle B A B prime is a right angle.

Visual / Anchor Chart

Standards

Building On
8.G.1

8.G.A.1

8.G.1

8.G.A.1

8.G.1

8.G.A.1

Addressing
8.G.3

8.G.A.3

8.G.3

8.G.A.3

Building Toward
8.G.3

8.G.A.3