Head First 2D Geometry

202 Chapter 4

your geometry toolbox

Your Geometry Toolbox

You’ve got Chapter 4 under

your belt and now you’ve

added properties of triangles

to your toolbox. For a complete

list of tool tips in the book, head to

www.headfirstlabs.com/geometry.

CHAPTER 4

Area =

1/2 base x height

The height comes from the

altitude of the triangle—it’s

perpendicular to the base.

Perimeter

Add up all the sides of

your shape to find the

perimeter.

Hero’s formula

Area = s (s-a) (s - b) (s - c)

s = a + b + c

2

Find the semiperimeter first,

then use it to find area.

Centroid

The point where all three

medians of the triangle

intersect

Median

A line from the midpoint of a

side to the opposite vertex

Incenter

The center of a circle drawn

inside the triangle, touching

every side

Circumcenter

The center of a circle drawn

outside the triangle, through

every vertex

Orthocenter

The point where all three

altitudes of the triangle

intersect

you are here 4 203

triangle properties

TriangleGrid Solution

Stop the press! Your buddies have secretly lined up

a headline act for the festival. Solve the grid puzzle

to be the rst to know who the mystery band are.

Clues

1. You use this theorem to find missing sides of right triangles.

2. Two angles adding up to 90º are known as this.

3. The month before August.

4. When lines cross, they create ...