Congruent triangles and similar shapes
Congruent shapes are identical twins; similar shapes are the same photo printed at different sizes.
Part of your national curriculum
- Geometry and measures: Identify and construct congruent triangles, and construct similar shapes by enlargement, with and without coordinate grids
Lesson overview
What you'll learn in this lesson
Identify and construct congruent triangles, and construct similar shapes by enlargement, with and without coordinate grids
Key learning points
- • Congruence
- • Similarity and enlargement
- • Worked example
This lesson at a glance
- 26 minutes
- 17 parts to scroll through
- 3 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 17
Visual introductionPicture this
Congruent triangles and similar shapes
Congruent shapes are identical twins; similar shapes are the same photo printed at different sizes.
In a nutshell
Identify and construct congruent triangles, and construct similar shapes by enlargement, with and without coordinate grids
Learning cycle
Part 2 of 17
Learning cycle 1 of 2
Part 1 · Congruence
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 17
Learn
Congruence
Two triangles are congruent if they are exactly the same size and shape, matched by rules such as SSS (three sides equal) or SAS (two sides and the included angle equal).
Reset break
Part 4 of 17
Pause
That's sitting 1 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 5 of 17
Learn
Similarity and enlargement
Similar shapes have the same angles and proportional sides, but different sizes. An enlargement by scale factor k multiplies every length by k while keeping angles unchanged.
Quick check
Part 6 of 17
Quick check
Part 7 of 17
Quick check
A triangle with sides 3, 4, 5 cm is enlarged by scale factor 2. What are the new sides?
Reset break
Part 8 of 17
Pause
That's sitting 2 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 9 of 17
Learning cycle 2 of 2
Part 2 · Worked example
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 17
Learn
Worked example
A triangle with sides 3 cm, 4 cm, 5 cm is enlarged by scale factor 2. The new sides are 3×2=6 cm, 4×2=8 cm, 5×2=10 cm — similar to the original, with all angles the same.
Quick check
Part 11 of 17
Reset break
Part 12 of 17
Pause
That's sitting 3 of 4 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 13 of 17
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 14 of 17
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Two triangles are ____ if they are exactly the same size and shape, matched by rules such as SSS (three sides equal) or SAS (two sides and the included angle equal).
____ shapes have the same angles and proportional sides, but different sizes.
Challenge round
Part 15 of 17
Game · Recall cards
Which condition proves two triangles are congruent?
Card 1 of 4
Mastery quiz
Part 16 of 17
Marked quiz
End of lesson quiz: Congruent triangles and similar shapes
4 questions, marked with the reasoning shown. No timer.
1. Which condition proves two triangles are congruent?
2. A triangle with sides 3, 4, 5 cm is enlarged by scale factor 2. What are the new sides?
3. What stays the same between similar shapes?
4. Which is NOT a congruence condition?
Lesson round-up
Part 17 of 17
Lesson round-up
Ready when you are
Quiz score
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Luna: 0 out of 3 on the practice checks. Only if you feel up to it — one more?
Ask LunaPart 1 of 17 · Watch & discover
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Identify and construct congruent triangles, and construct similar shapes by enlargement, with and without coordinate grids