Head First 2D Geometry

a

b

c

b

The dark triangle is turned 60º compared to the

light one, and each angle of the triangle is 60º so

these edges are parallel. A line which is perpendicular

to one line is also perpendicular to lines parallel to

that line, so b is also 90º (another 6).

88 Chapter 2

exercise solution

a

c

To find angle c, draw a

line parallel to the base of

the dark triangle.

60º

This creates some equal angles—the Z pattern. Angle c is

this Z angle (60º) plus the perpendicular (it’s one side of

the square and we drew our line parallel to the bottom of

the square), so

c = 60º + 90º = 150º (6 of these, too!)

48 down, 12 to go....

54 down…just six left…

a

150º

To find angle a, first find the angles of this triangle.

This angle is the supplement of a right angle we already found, so it’s 90º.

This angle is the supplement of the 150ºwe just found, so it’s 30º.

The angle next to a is the remaining angle in the triangle = 60º, so a is the

supplement of this angle = 180º- 60º = 120º (6 of these as well).

…continued

you are here 4 89

similarity and congruence

Tonight’s talk: Similarity and Congruence

Similarity:

I’m worried about you. You seem neurotic, always

concerned about being a size zero, or seven, or

whatever. You need to relax more.

I just think if you stopped worrying so much about

size and focused on proportionality ...