Circles

You already know that a circle has 360° . Circles can be divided into different sectors: a half circle or semi-circle has 180° , a quarter circle has 90° , and an eighth of a circle has 45° .

Circles are covered in more detail elsewhere in the book.

Triangles

A triangle is a closed figure that has three sides and three angles.

The sum of the three angles of every triangle always equals 180° .

A right triangle always has one angle equal to 90° and two angles whose sum is 90° , making a total of 180° .

A rectangle is a four sided shape
with four right angles (90° ).

The sides across from each other are always
the same length and always parallel to
each other.

The four angles add up to a total of 360° .

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Another important rule about rectangles is that the diagonals of a rectangle are equal.

Diagonals are lines that run between opposite corners.

A diagonal always divides a rectangle exactly in half and creates two equal right triangles.

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Carpenters doing layout work on forms
measure both diagonals to make sure they
are the same length. This insures that their
layout is a rectangle which has square
corners.

Squares

The square is a special type of rectangle in which all sides
are equal. Since the square is a rectangle, the same rules
that apply to the rectangle also apply to the square.

A diagonal across the square creates two equal
45° right triangles.

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Later, you will be working with
45° right triangles.

It will be helpful to remember
that the 45° right triangle is
half of a square.