Big Ideas Math: Modeling Real Life, Grade 8
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Big Ideas Math: Modeling Real Life, Grade 8 View details
4. Congruent Figures
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Exercise 2 Page 67

When a point with coordinates (x,y) is rotated 180^(∘) clockwise about the origin, the coordinates of its image are (- x,- y).

F'(2,- 1), G'(1,- 3), H'(- 3,- 1)

Practice makes perfect

A rotation is a transformation about a fixed point called center of rotation. Each point of the original figure and its image are the same distance from the center of rotation. When a clockwise rotation is performed about the origin, the coordinates of the image can be written in relation to the coordinates of the preimage.

Rotations About the Origin
90^(∘) Rotation 180^(∘) Rotation 270^(∘) Rotation

ccc Preimage & & Image [0.5em] (x,y) & → & (y,- x)

ccc Preimage & & Image [0.5em] (x,y) & → & (- x,- y)

ccc Preimage & & Image [0.5em] (x,y) & → & (- y,x)

We want to rotate a triangle 180^(∘) clockwise about the origin. Therefore, we can use the information in the above table to find the coordinates of the image of each vertex.

Preimage (x,y) Image (- x, - y) Simplify
F(- 2,1) F(-(- 2),-1) F'(2, - 1)
G(- 1,3) G(-(- 1),- 3) G'(1, - 3)
H(3,1) H(- 3,- 1) H'(- 3,- 1)
We can now plot the obtained points and draw the image of the given triangle after the rotation!
preimage and image

Extra

Visualizing the Rotation
Let's rotate FGH 180^(∘) clockwise about the origin so that we can see how it is mapped onto F'G'H'.
rotate