Head First 2D Geometry

you are here 4 19

finding missing angles

Vertical angles are always equal

When two straight lines cross they always create two pairs of

equal opposite angles, called vertical angles.

Each angle has to form a supplementary pair with either of

the angles on the other side of it, so they must be equal.

These two

are equal.

These two are

also equal.

So if you know

this one…

…you already know

this one, too!

Save yourself a ton of

math by spotting equal

pairs of vertical angles.

Patterns in geometry aren’t just coincidence

These relationships are how geometry works…and to see a

really freaky one, try this:

On some scrap paper, draw a triangle using a ruler.

Cut the triangle out.

Tear the corners off.

And then put the three corner pieces together with

the points in the middle.

Weird, huh?

Do this!

Two straight

lines cross

Add either to the original

angle and you get 180º.

Pick one

angle

The supplements

must be equal

Find its

supplements

&

&

This is one

supplement…

…and here’s the

other one.