angle. A straight angle can be thought of
like a book opened flat on a desk. An
angle opened half that far is called a
right angle.
Angles are measured in degrees. A right
angle measures 90 degrees. Angles that
are smaller than a right angle are called
acute. Angles that are larger than a right
angle but smaller than a straight angle
are called obtuse.
Polygons
A polygon is a closed figure formed by
joining together line segments. A polygon
with three sides is a triangle. Some
kinds of triangles have special names. If
all three sides of a triangle are equal, it is
an equilateral triangle. A triangle with
one right angle is a right triangle.
A polygon with four sides is called a
quadrilateral. A rectangle is a quadrilateral
with sides that form four right
angles. A square is a special kind of rectangle
with four sides of equal length.
Polygons with more than four sides are
named by combining Greek words for
numbers with the letters “-gon.” For
example, a pentagon is a five-sided fig-
Petrology and
petrography
are branches
of geology
that deal with
the study of
rocks.
98 Geometry BRITANNICA STUDENT ENCYCLOPEDIA
ure. A hexagon has six sides, and an
octagon has eight.
Circles
Some geometric shapes have curves
instead of straight lines. The most familiar
of these shapes is the circle. All the
points on the edge of a circle are the
same distance from the center. This distance
is called the radius. The diameter
is the distance across a circle through the
center. It is twice the length of the
radius. The length around the outside of
the circle is called the circumference.
Solid Figures
Triangles, squares, and circles are flat.
Other geometric shapes are solid figures.
A cube is a solid figure with flat surfaces.
A square box is an example of a cube. A
sphere, such as a baseball, is a solid figure
with a curved surface. Cones, cylinders,
and pyramids are other solid
figures.
#More to explore
Mathematics
George III
King George III ruled Great Britain
during the time of the American Revolution.
Problems with the colonies had
started before George became king.
Even so, people remember George III as
the king who lost the American colonies.
Early Life
GeorgeWilliam Frederick was born in
London, England, on May 24, 1738. He
became king when his grandfather, King
George II, died in 1760. George III’s
father had died earlier.
Losing the Colonies
By the time George became king, British
monarchs (kings and queens) had lost
much of their power to Parliament. Parliament
was the group of lawmakers in
Britain. Beginning in the 1760s Parlia-
In 1761
George married
Charlotte
Sophia of
Mecklenburg-
Strelitz. They
had 15
children.
BRITANNICA STUDENT ENCYCLOPEDIA George III 99
ment tried to raise money by taxing the
American colonies. The colonists complained
that Parliament had no right to
tax them because they had no representatives
in it.
In 1775 George announced that the
colonists were committing treason, the
crime of betraying one’s country. In the
same year, the colonists began fighting
for independence. In the U.S. Declaration
of Independence, Thomas Jefferson
attacked George and called him a tyrant
(a cruel, lawless ruler). The colonies won
independence from Britain in 1783.
Illness and Death
After the American Revolution, George
became very ill. Modern doctors think
that he may have suffered from a disease
called porphyria. His disease made him
hallucinate (see things that were not
there). At the time, people called his
condition madness.
In 1811 his son Prince George took over
the throne as regent, or substitute king.
George III died on January 29, 1820.
Prince George became King George IV.
#More to explore
American Revolution
Georgetown
Population
(2002 estimate)
137,330
Georgetown is the capital of Guyana, a
country on the northern coast of South
America. The city lies on the Atlantic
Ocean. It is Guyana’s only large city.
Georgetown is the country’s center of
business and manufacturing. It is also
the main port. Sugar processing is a
major industry in the city.
The British founded Georgetown in
1781. They named the settlement after
King George III of England. French and
later Dutch settlers took over the town.
The British regained control in 1812. In
1831 they made all of Guyana into a
British colony. Guyana became an independent
country in 1966. Georgetown
was made its capital.
#More to explore
Guyana
King George III wears his military
uniform in a portrait from
about 1800.
100 Georgetown BRITANNICA STUDENT ENCYCLOPEDIA
Georgia
The U.S. state of Georgia is called the
Empire State of the South. This nickname
reflects Georgia’s large size and
economic strength. Georgia is as important
to the South as New York (the
Empire State) is to the Northeast. The
state was named for King George II of
England. In 1732 the king granted permission
for the area to become a colony.
It is the only United States state with a
name honoring an English king. The
capital is Atlanta.
Geography
Georgia is bordered on the south by
Florida, on the east by the Atlantic
Ocean and South Carolina, on the north
by North Carolina and Tennessee, and
on the west by Alabama. The Savannah
