Lesson · Grade 5 · Mathematics
Reflection: Flipping a Shape Over a Line
- Grade 5
- Mathematics
- 1 min 10 s
- British Columbia
- BC5-MA-33
Triangle ABC is reflected in a mirror line on a square grid by counting straight across: A, B and C are 4, 2 and 5 squares from the line, so A′, B′ and C′ land 4, 2 and 5 squares past it. Folding along the mirror line checks that the triangle lands exactly on its image, which is the same size and shape but faces the other way, and the video ends with the rule: move each corner the same distance past the line.
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In one sentence
The question this lesson answers
A reflection flips a shape over a line, called the mirror line.
Where will triangle A B C land when we flip it over the line?
Transcript
What the lesson says
A reflection flips a shape over a line, called the mirror line.
Where will triangle A B C land when we flip it over the line?
Count straight across: corner A is 4 squares from the line.
Count 4 squares past the line to find A prime.
B is 2 squares from the line, so B prime is 2 squares past it.
C is 5 squares from the line, so C prime is 5 squares past it.
Join A prime, B prime and C prime to draw the image.
To check, fold the grid along the mirror line.
The triangle lands exactly on its image.
The image is the same size and the same shape as triangle A B C.
But this image faces the other way, like your reflection in a mirror.
To reflect a shape, move each corner straight across, the same distance past the line.
Curriculum
Where this fits
- British Columbia, Grade 5
Mathematics, Geometry and measurement: single transformationsBC5-MA-33
Previous: Translation: Sliding a Shape on a GridNext: Rotation: Quarter, Half and Three-Quarter Turns
The outcomes and strands follow the British Columbia curriculum.
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