Learn on PengiBig Ideas Math, Course 2, AcceleratedChapter 1: Transformations

Lesson 2: Translations

In this Grade 7 lesson from Big Ideas Math Course 2 Accelerated, students learn how to identify translations and perform them in the coordinate plane by sliding figures a given distance and direction without changing their size, shape, or orientation. Students explore how repeated translations of a single tile can produce tessellations, patterns that cover a plane with no gaps or overlaps. The lesson also has students compare corresponding vertices, side lengths, and angle measures of original and translated figures to establish congruence.

Section 1

Defining a Translation

Property

A translation is a rigid transformation that "slides" a figure across a plane to a new location. Every single point of the original figure (the pre-image) moves the exact same distance and in the exact same direction to create the new figure (the image). Because it is a rigid motion, the figure does not rotate, reflect, or change its size. Therefore, the pre-image and image are perfectly congruent and face the exact same way (they preserve orientation).

Examples

  • Macro View: Sliding a physical ruler across your desk without rotating it.
  • Micro Detail (Naming): When triangle ABC slides to a new position, the new triangle is named A'B'C' (read as "A prime, B prime, C prime"). Point A matches with A', B with B', and C with C'.
  • Micro Detail (Direction): If you draw a straight line from A to A' and another from B to B', those lines will be perfectly parallel and the exact same length.

Explanation

While the property tells us the shape just "slides," here are the micro-details to watch out for:

  1. Pre-image vs. Image: The original starting shape is called the "pre-image" (usually standard letters like A, B, C). The final landing spot is the "image" (indicated by the prime marks like A', B', C').
  2. Congruence: Because it's a "rigid" motion, the pre-image and image are exactly identical. If the side length of AB was 5 units, the side length of A'B' is strictly 5 units. No stretching allowed!

Section 2

Translations Preserve Size and Shape

Property

A translation is a rigid motion, meaning it preserves the size and shape of a figure. The translated figure (the image) is always congruent to the original figure (the preimage).

If figure FF is translated to create image F′F', then F≅F′F \cong F'. This means all corresponding side lengths and angle measures are equal.

Section 3

Writing a Coordinate Rule for a Translation

Property

The visual slide of a translation is written as an algebraic coordinate rule: (x, y) -> (x + a, y + b).

  • (x, y) represents any starting point on the pre-image.
  • "a" is the horizontal shift added to the x-coordinate (use +a for Right, -a for Left).
  • "b" is the vertical shift added to the y-coordinate (use +b for Up, -b for Down).

This single rule acts as a master instruction that applies identically to every point on the figure.

Examples

  • Translating Words to Rule: "Left 4, Up 5" becomes (x, y) -> (x - 4, y + 5).
  • Handling Zeroes: "Right 3, but no vertical movement" becomes (x, y) -> (x + 3, y). Notice we don't write y + 0, just y.
  • Finding the Rule from Math: If M(2, 5) moves to M'(7, 1):
    • x-change: 7 - 2 = +5
    • y-change: 1 - 5 = -4
    • Rule: (x, y) -> (x + 5, y - 4).

Explanation

Let's look at the hidden mechanics of this formula:

  1. The Arrow (->): This symbol means "maps to" or "becomes". It separates the "before" (x, y) from the "after" (x + a, y + b).
  2. It's a Master Template: The rule (x, y) -> (x + 2, y - 3) doesn't mean x equals 2. It means "take whatever your starting x-coordinate is, and add 2 to it." You apply this identical template to every single vertex of your shape.

Lesson overview

Expand to review the lesson summary and core properties.

Expand

Section 1

Defining a Translation

Property

A translation is a rigid transformation that "slides" a figure across a plane to a new location. Every single point of the original figure (the pre-image) moves the exact same distance and in the exact same direction to create the new figure (the image). Because it is a rigid motion, the figure does not rotate, reflect, or change its size. Therefore, the pre-image and image are perfectly congruent and face the exact same way (they preserve orientation).

Examples

  • Macro View: Sliding a physical ruler across your desk without rotating it.
  • Micro Detail (Naming): When triangle ABC slides to a new position, the new triangle is named A'B'C' (read as "A prime, B prime, C prime"). Point A matches with A', B with B', and C with C'.
  • Micro Detail (Direction): If you draw a straight line from A to A' and another from B to B', those lines will be perfectly parallel and the exact same length.

Explanation

While the property tells us the shape just "slides," here are the micro-details to watch out for:

  1. Pre-image vs. Image: The original starting shape is called the "pre-image" (usually standard letters like A, B, C). The final landing spot is the "image" (indicated by the prime marks like A', B', C').
  2. Congruence: Because it's a "rigid" motion, the pre-image and image are exactly identical. If the side length of AB was 5 units, the side length of A'B' is strictly 5 units. No stretching allowed!

Section 2

Translations Preserve Size and Shape

Property

A translation is a rigid motion, meaning it preserves the size and shape of a figure. The translated figure (the image) is always congruent to the original figure (the preimage).

If figure FF is translated to create image F′F', then F≅F′F \cong F'. This means all corresponding side lengths and angle measures are equal.

Section 3

Writing a Coordinate Rule for a Translation

Property

The visual slide of a translation is written as an algebraic coordinate rule: (x, y) -> (x + a, y + b).

  • (x, y) represents any starting point on the pre-image.
  • "a" is the horizontal shift added to the x-coordinate (use +a for Right, -a for Left).
  • "b" is the vertical shift added to the y-coordinate (use +b for Up, -b for Down).

This single rule acts as a master instruction that applies identically to every point on the figure.

Examples

  • Translating Words to Rule: "Left 4, Up 5" becomes (x, y) -> (x - 4, y + 5).
  • Handling Zeroes: "Right 3, but no vertical movement" becomes (x, y) -> (x + 3, y). Notice we don't write y + 0, just y.
  • Finding the Rule from Math: If M(2, 5) moves to M'(7, 1):
    • x-change: 7 - 2 = +5
    • y-change: 1 - 5 = -4
    • Rule: (x, y) -> (x + 5, y - 4).

Explanation

Let's look at the hidden mechanics of this formula:

  1. The Arrow (->): This symbol means "maps to" or "becomes". It separates the "before" (x, y) from the "after" (x + a, y + b).
  2. It's a Master Template: The rule (x, y) -> (x + 2, y - 3) doesn't mean x equals 2. It means "take whatever your starting x-coordinate is, and add 2 to it." You apply this identical template to every single vertex of your shape.