Head First 2D Geometry

finding missing angles

Find one more angle to crack the case

There’s only one unknown angle left on the diagram. Once you’ve

figured out this one you can charge Benny with the shooting for sure.

Find this last mystery angle to complete your investigation

and prove that Benny was the shooter.

How does it fit in with what you’ve already found out?

CRIME SCENE SKETCH

CASE GEO180

(ANGLES AS MEASURED

BUT DRAWING NOT TO

SCALE)

BULLET

PA

TH

MICKY

BENNY

18º

81º

86º

72º

9º

81º 99º

?º

FOOTPRINTS

you are here 4 21

Hmm…something f

unn

y

is going on. It doesn’t

ma

tter how you w

orded

i

t,.as long as y

ou noticed

something didn’t add up.

something funny is going on…

Find this last mystery angle to complete your investigation and

prove that Benny was the shooter.

How does it fit in with what you’ve already found out?

CRIME SCENE SKETCH

CASE GEO180

(ANGLES AS MEASURED

BUT DRAWING NOT TO

SCALE)

BULLET

PA

TH

MICKY

BENNY

FOOTPRINTS

18º

81º

86º

72º

9º

81º 99º

?º

There are 2 different ways to find this

mystery angle—you might have done either one.

1)

2)

Angle c is opposite

this angle here

Angles in a triangle add up to 180º

And vertical angles are equal,

so 81º + 86º + ? = 180º

so ? = 18º

? = 180º - 81º - 86º = 13º

But this doesn’t fit with the

triangle angles:

But this doesn’t fit with the 18º

angle it’s opposite. They should be

equal.

18º + 81º + 86º = 185º

They should add up to 180º.

22 Chapter 1