Head First 2D Geometry

sharpen solution

30 Chapter 1

TIRE

TRACKS

x and y are supplementary

x is less than y

x is equal to y

nothing—they could be any angles

xº

yº

Tick marks indicate equal angles

Tick marks on angles are used to show that the

angles are equal, even if we don’t know what size

the angles are.

Each set of equal angles has a different number of

ticks, so you can mark more than one set on the

same sketch if you need to.

Things that bounce equally

bounce like this.

bounce

These tick marks on

the angles show that

they are equal.

Remember—your sketch isn’t

usually accurate enough to

see by eye whether two

angles are equal.

Unless you’re told otherwise, a

sketch is just a sketch. Even if the angles drawn

look pretty close to the real angles, you can’t rely

on your eye or a protractor—you’ve gotta work

the angles out.

If our bullet bounced as the lab technician

desribed, what do we know should be true

about mystery angles x and y?