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The Pythagorean Theorem
Objectives
Use the Pythagorean Theorem and its
converse to solve problems.
Use Pythagorean inequalities to classify
triangles.
Holt McDougal Geometry
The Pythagorean Theorem
Vocabulary
Pythagorean triple
Holt McDougal Geometry
The Pythagorean Theorem
The Pythagorean Theorem is probably the most
famous mathematical relationship. As you learned in
Lesson 1-6, it states that in a right triangle, the sum
of the squares of the lengths of the legs equals the
square of the length of the hypotenuse.
a2 + b2 = c2
Holt McDougal Geometry
The Pythagorean Theorem
Example 1A: Using the Pythagorean Theorem
Find the value of x. Give
your answer in simplest
radical form.
a2 + b2 = c2
Pythagorean Theorem
22 + 62 = x2
Substitute 2 for a, 6 for b, and x for c.
40 = x2
Simplify.
Find the positive square root.
Simplify the radical.
Holt McDougal Geometry
The Pythagorean Theorem
Example 1B: Using the Pythagorean Theorem
Find the value of x. Give
your answer in simplest
radical form.
a2 + b2 = c2 Pythagorean Theorem
(x – 2)2 + 42 = x2 Substitute x – 2 for a, 4 for b, and x for c.
x2 – 4x + 4 + 16 = x2 Multiply.
–4x + 20 = 0 Combine like terms.
20 = 4x Add 4x to both sides.
5=x
Holt McDougal Geometry
Divide both sides by 4.
The Pythagorean Theorem
Check It Out! Example 2
What if...? According to the
recommended safety ratio of 4:1, how
high will a 30-foot ladder reach when
placed against a wall? Round to the
nearest inch.
Let x be the distance in feet from the foot of the
ladder to the base of the wall. Then 4x is the
distance in feet from the top of the ladder to the
base of the wall.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 2 Continued
a2 + b2 = c2
(4x)2
+
x2
=
302
17x2 = 900
Pythagorean Theorem
Substitute 4x for a, x for b, and
30 for c.
Multiply and
combine like terms.
Since 4x is the distance in feet from the top of the
ladder to the base of the wall, 4(7.28)  29 ft 1 in.
Holt McDougal Geometry
The Pythagorean Theorem
A set of three nonzero whole numbers a, b,
and c such that a2 + b2 = c2 is called a
Pythagorean triple.
Holt McDougal Geometry
The Pythagorean Theorem
The converse of the Pythagorean Theorem gives you
a way to tell if a triangle is a right triangle when you
know the side lengths.
Holt McDougal Geometry
The Pythagorean Theorem
You can also use side lengths to classify a
triangle as acute or obtuse.
B
c
A
Holt McDougal Geometry
a
b
C
The Pythagorean Theorem
To understand why the Pythagorean inequalities are
true, consider ∆ABC.
Holt McDougal Geometry
The Pythagorean Theorem
Remember!
By the Triangle Inequality Theorem, the sum of
any two side lengths of a triangle is greater
than the third side length.
Holt McDougal Geometry
The Pythagorean Theorem
Example 4A: Classifying Triangles
Tell if the measures can be the side lengths
of a triangle. If so, classify the triangle as
acute, obtuse, or right.
5, 7, 10
Step 1 Determine if the measures form a triangle.
By the Triangle Inequality Theorem, 5, 7, and 10
can be the side lengths of a triangle.
Holt McDougal Geometry
The Pythagorean Theorem
Example 4A Continued
Step 2 Classify the triangle.
c2
102
?
= a2 + b2
?
= 52 + 72
?
Compare c2 to a2 + b2.
Substitute the longest side for c.
100 = 25 + 49
Multiply.
100 > 74
Add and compare.
Since c2 > a2 + b2, the triangle is obtuse.
Holt McDougal Geometry
The Pythagorean Theorem
Example 4B: Classifying Triangles
Tell if the measures can be the side lengths
of a triangle. If so, classify the triangle as
acute, obtuse, or right.
5, 8, 17
Step 1 Determine if the measures form a triangle.
Since 5 + 8 = 13 and 13 > 17, these cannot be
the side lengths of a triangle.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 4a
Tell if the measures can be the side lengths of
a triangle. If so, classify the triangle as acute,
obtuse, or right.
7, 12, 16
Step 1 Determine if the measures form a triangle.
By the Triangle Inequality Theorem, 7, 12, and
16 can be the side lengths of a triangle.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 4a Continued
Step 2 Classify the triangle.
c2
162
?
= a2 + b2
?
= 122 + 72
?
Compare c2 to a2 + b2.
Substitute the longest side for c.
256 = 144 + 49
Multiply.
256 > 193
Add and compare.
Since c2 > a2 + b2, the triangle is obtuse.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 4b
Tell if the measures can be the side lengths of
a triangle. If so, classify the triangle as acute,
obtuse, or right.
11, 18, 34
Step 1 Determine if the measures form a triangle.
Since 11 + 18 = 29 and 29 > 34, these
cannot be the sides of a triangle.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 4c
Tell if the measures can be the side lengths of
a triangle. If so, classify the triangle as acute,
obtuse, or right.
3.8, 4.1, 5.2
Step 1 Determine if the measures form a triangle.
By the Triangle Inequality Theorem, 3.8, 4.1, and
5.2 can be the side lengths of a triangle.
Holt McDougal Geometry
The Pythagorean Theorem
Check It Out! Example 4c Continued
Step 2 Classify the triangle.
c2
5.22
?
= a2 + b2
?
= 3.82 + 4.12
Compare c2 to a2 + b2.
Substitute the longest side for c.
?
27.04 = 14.44 + 16.81 Multiply.
27.04 < 31.25
Add and compare.
Since c2 < a2 + b2, the triangle is acute.
Holt McDougal Geometry