Head First 2D Geometry

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similarity and congruence

Use what you know to find what you don’t know

We said it in Chapter 1, and it still applies now. Work from what

you do know to find out what you don’t know. Like the angles

between the arrows.

And you know this angle here

(it’s an equilateral triangle

because those three arrows are

congruent).

And you know these

two angles here…

This angle completes

a 360º “whole turn.”

And if you don’t have what you need, add it!

You can add parallel or perpendicular lines to your

sketch to break down the missing angles into parts you

have the tools to find.

And once you’ve found

it, you can just copy it

to the other angles it is

congruent with.

Adding a line

here creates a

Z pattern.

These two lines are

parallel (because we

say so!)

Then we can use

what we know about

these angles…

…to find

these angles.

Ready to kick some

serious design butt?

find all the angles

84 Chapter 2

There are 60 angles on the band’s logo design. Use the space on the right to start working on

the sketch and calculate them all. How many of each different angle are there?

Feeling overwhelmed?

Don’t panic! Everything

you need is in your

Chapter 1 toolbox.

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similarity and congruence ...