6.2 The Pythagorean Theorems
One of the best known theorems in geometry (and all of mathematics for that matter) is the
Pythagorean Theorem. You have probably already worked with this theorem in your algebra
studies. The theorem is attributed to Pythagoras, a Greek mathematician and philospher, who
p
roved this theorem about twentyfive hundred years ago.
In order to understand the proof we must first investigate geometric means.
Proportions in which the means are equal occur frequently in geometry. For any two positive
numbers a and b, the geometric mean, of a and b is the positive number x such that
a
ÅÅ
Å
Å
x
=
x
ÅÅ
Å
Å
b
.
Example 1: Find the geometric mean of 4 and 8.
Solution:
4
Å
Å
Å
Å
x
=
x
Å
Å
Å
Å
8
Write the proportion
x
2
= 32 Use the CrossProduct Property
x =
è!!!!!!
32 Find the positive square root
x = 4
è!!!
2 Write in simplest radical form
The following theorem shows us how several geometric means "pop up" when you draw the
altitude to the hypotenuse of a right triangle.
Theorem: The altitude to the hypotenuse of a right triangle divides the triangle into two
triangles that are similar to the original triangle and to each other.
.
PythagoreanTheorems20052006.nb 1
Look at the figure below. We are given right DABC with CD
ê
êêê
ê
as the altitude to the hypotenuse.
A B
C
D
DABC ~ DACD ~ DCBD
This theorem can be easily proved by the AA Similarity Postulate since both smaller triangles
share an acute angle with DABC and all three triangles are right triangles. Since both smaller
triangles are similar to DABC, their corresponding angles are congruent. Thus the two smaller
triangles are similar to each other.
There are two important corollaries of this theorem one of which will be used in the proof of
the Pythagorean Theorem.
Corollary 1: When the altitude is drawn to the hypotenuse of a right triangle, the length of
the altitude is the geometric mean between the segments of the hypotenuse.
Corollary 2: When the altitude is drawn to the hypotenuse of a right triangle, each leg is
the geometric mean between the hypotenuse and the segment of the hypotenuse that is adja
cent to the leg.
PythagoreanTheorems20052006.nb 2
Let's look at our triangle again.
A B
C
D
The first corollary states that CD is the geometric mean of AD and DB. Thus,
AD
ÅÅÅ
Å
Å
Å
Å
Å
Å
C
D
=
CD
ÅÅÅ
Å
Å
Å
Å
Å
Å
DB
.
The second corollary states that AC is the geometric mean of AB and AD (the segment of the
hypotenuse adjacent to AC) and that CB is the geometric mean of AB and DB. Thus,
AD
ÅÅÅ
Å
Å
Å
Å
Å
Å
A
C
=
AC
ÅÅÅ
Å
Å
Å
Å
Å
Å
AB
and
DB
ÅÅÅ
Å
Å
Å
Å
Å
Å
C
B
=
CB
Å
ÅÅ
Å
ÅÅ
Å
Å
Å
AB
N
ow we are ready to state and prove the Pythagorean Theorem.
Pythagorean Theorem: In a right triangle, the square of the hypotenuse is equal to the
sum of the squares of the legs.
PythagoreanTheorems20052006.nb 3
Given: Right DABC; —C is a right —.
Prove: c
2
= a
2
+b
2
A B
C
ab
d e
c
Proof:
Statements Reasons
1. Draw a perpendicular from C to AB
êêêê
ê
1. Through a point outside a line, there is
exactly one line perp. to the line.
2.
c
Å
Å
Å
Å
a
=
a
Å
Å
Å
Å
e
;
c
Å
Å
Å
Å
b
=
b
Å
Å
Å
Å
d
2. Corollary 2 from above.
3. ce = a
2
; cd = b
2
3. A property of proportions
4. ce + cd = a
2
+b
2
4. Addition property
5. c(e + d) = a
2
+ b
2
5. Distributive property
6. c
2
= a
2
+b
2
6. Substitution property
Let's look at some examples.
PythagoreanTheorems20052006.nb 4
Example 2: Find the value of x.
3
6
x
Solution: x
2
= 6
2
+3
2
x
2
= 36 +9
x
2
= 45
x =
è!!!!!!
45
x = 3
è!!!
5 Note: we only used the positive square root
since x represents a length.
Example 3: Find the value of x.
x+2
x
10
Solution: x
2
+Hx +2L
2
= 10
2
x
2
+ x
2
+4
x +4 = 100
2
x
2
+4
x 96 = 0
x
2
+2
x 48 = 0
(x + 8)(x  6) = 0
x = 8 or x = 6. Since x represents a length we discard the negative solution
and x = 6.
PythagoreanTheorems20052006.nb 5
The converse of the Pythagorean Theorem is also true and we present it, without proof, below.
The Converse of the Pythagorean Theorem: If the square of one side of a triangle is
equal to the sum of the squares of the other two sides, then the triangle is a right triangle.
Example 4: Determine if the triangle with side lengths 5, 12, and 13 is a right triangle.
Solution: 5
2
+12
2
= 25 +144 = 169 and 13
2
= 169 hence
5
2
+12
2
= 13
2
and by the converse of the Pythagorean Theorem the triangle is a right triangle.
Three integers (like 5,12, and 13) that satisfy the conditions of the Pythagorean Theorem are
called Pythagorean Triples. If the three integers are relatively prime (meaning they have no
common factors) then the three integers are know and Primitive Pythagorean Triples. Some
common Primitive Pythagorean Triples and their multiples are listed below. Memorizing
some of these triples can make your computations easier in the future.
Some Common Pythagorean Triples
3,4,5 5,12,13 8,15,17 7,24,25
6,8,10
9,12,15
12,16,20
10,24,26
When a triangle is not a right triangle, the squares of the sides can be used to determine
whether the triangle is obtuse or acute. Look at the theorems below.
Theorem: If the square of the longest side of a triangle is greater than the sums of the
squares of the other two sides, then the triangle is an obtuse triangle.
PythagoreanTheorems20052006.nb 6
A little common sense can tell us why this is true as well. Picture a right triangle.
a
b
c
The conclusion of the Pythagorean Theorem tells us that c
2
= a
2
+b
2
. Now picture the right
angle opening like a door without changing lengths a and b.
a
b
d
Obviously, the new longest side, d, is longer than the length of c in the first triangle. So, it
makes sense that d
2
> c
2
and thus d
2
> a
2
+b
2
.
Theorem: If the square of the longest side of a triangle is less than the sum of the squares
of the other two sides, then the triangle is an acute triangle.
N
ow picture the right angle closing.
a
b
e
PythagoreanTheorems20052006.nb 7
Obviously, in this case, e < c and thus e
2
< c
2
and e
2
< a
2
+b
2
. So we now have a way to
determine if a triangle is a right triangle, an obtuse triangle, or an acute triangle, if we know
the lengths of all three sides of the triangle.
Example 5: Determine whether a triangle formed with sides having the lengths named is
acute, right, or obtuse.
(A) 9, 40, 41 (B) 6, 7, 8 (C) 8, 10, 14
Solutions:
(A) 9
2
+40
2
?
41
2
81 + 1600
?
1681
1681 = 1681
The triangle is right.
(B) 6
2
+7
2
?
8
2
36 + 49
?
64
85 > 64
The triangle is acute.
(C) 8
2
+10
2
?
14
2
64 + 100
?
196
164 < 196
The triangle is obtuse.
N
ow we are ready to look at some special right triangles in the next section.
PythagoreanTheorems20052006.nb 8
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