Similar triangles and congruent triangles
Watch the whole lecture free
Make a free account to watch every lecture to the end: 8 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
That was the teaching part. The rest of this lecture is free with an account: 8 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (1 min) is free to watch. A free account opens the whole lecture: 8 practice questions, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Shrink it, same shape
- Similar shapes
- Finding a missing side
Read the transcript of the teaching part
Last time
Before we start: last time, in Similar shapes and congruent triangles, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Shrink it, same shape
Put a photo on a photocopier and enlarge it. The picture is bigger, but every angle is the same and every length is scaled by the same amount. Those two shapes are similar. If a second photo is an exact copy, same size and same shape, the shapes are congruent. Two ideas, one big difference.
Similar shapes
Two shapes are similar if all their angles are equal and all their sides are in the same ratio. The ratio is the scale factor. Triangle one has sides three, four, five. Triangle two has sides six, eight, ten. Each side doubled: the scale factor from the small to the large is two.
Finding a missing side
Triangles ABC and DEF are similar. A B is six and D E is nine. B C is eight. Find E F. The scale factor from the first to the second is nine over six, which is one point five. E F corresponds to B C, so E F is eight times one point five: twelve.
Chapter 20: Similarity and congruence
- Similar triangles and congruent triangles 5 min
- Areas and volumes of similar shapes 5 min
Also for IGCSE Maths 0580
Which topics come up most in IGCSE Maths 0580 past papers, counted from every paper since 2004.