1. The Pythagorean Theorem
Section 8-1

2. Objectives
Use the Pythagorean Theorem.

3. Key Vocabulary
Leg
Hypotenuse
Pythagorean Theorem
Pythagorean Triple

4. Parts of a Right Triangle
Longest side is the hypotenuse, side c (opposite the 90o angle).
The other two sides are the legs, sides a and b.
Pythagoras developed a formula for finding the length of the sides of any right triangle.

5. Theorem 4.7 - The Pythagorean Theorem
In a right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the legs.
Example:
(hypotenuse)2=(leg)2+(leg)2

6. Example 1 Find the length of the hypotenuse. SOLUTION
(hypotenuse)2 = (leg)2 + (leg)2
Pythagorean Theorem
c2 =
Substitute.
c2 =
Multiply.
c2 = 169
Add.
Find the positive square root.
c2 =
169
c = 13
Solve for c.
ANSWER
The length of the hypotenuse is 13. 6

7. Example 2 Find the unknown side length. SOLUTION
(hypotenuse)2 = (leg)2 + (leg)2
Pythagorean Theorem
142 = 72 + b2
Substitute.
196 = 49 + b2
Multiply.
196 – 49 = 49 + b2 – 49
Subtract 49 from each side.
147 = b2
Simplify.
Find the positive square root.
147 = b2
12.1 ≈ b
Approximate with a calculator.
ANSWER
The side length is about 12.1. 7

8. Your Turn: Find the unknown side length. 1. ANSWER 8 2. ANSWER 8 3.
about 10.6

9. Example 3a
A. Find x.
The side opposite the right angle is the hypotenuse, so c = x.
a2 + b2 = c2 Pythagorean Theorem
= c2 a = 4 and b = 7

10. Example 3a
65 = c2 Simplify.
Take the positive square root of each side.
Answer:

11. Example 3b B. Find x. The hypotenuse is 12, so c = 12.
a2 + b2 = c2 Pythagorean Theorem
x = 122 b = 8 and c = 12

12. Example 3b x2 + 64 = 144 Simplify. x2 = 80 Subtract 64 from each side.
Take the positive square root of each side and simplify.
Answer:

13. Your Turn:
A. Find x. A. B. C. D.

14. Your Turn:
B. Find x. A. B. C. D.

15. More Examples: C A B B = 6 1) A=8, C =10 , Find B
3) B =10, C=26 , Find A
4) A=15, B=20, Find C
5) A =12, C=16, Find B
6) B =5, C=10, Find A
7) A =6, B =8, Find C
8) A=11, C=21, Find B
B = 8
A = 24
C = 25 C
B = 10.6 A
A = 8.7
C = 10
B = 17.9 B

16. Pythagorean Triples
Three whole numbers that work in the Pythagorean formulas are called Pythagorean Triples. The largest number in each triple is the length of the hypotenuse. Pythagorean triples are not the only possible side lengths for a right triangle. They give the triangles where all the lengths are whole numbers, but the side lengths could be any real numbers.

17. Pythagorean Multiples
If you multiply the lengths of all three sides of any right triangle by the same number, then the resulting triangle is a right triangle.
In other words, if a2 + b2 = c2, then (an)2 + (bn)2 = (cn)2.
Therefore, additional pythagorean triples can be found by multiplying each number in a known triple by the same factor.

18. Pythagorean Triples
Multiples

19. Primitive Pythagorean Triples
A set of Pythagorean triples is considered a primitive Pythagorean triple if the numbers are relatively prime; that is, if they have no common factors other than 1. You need know the first 4 primitives: 3-4-5, , ,
3-4-5

20. Example 4
Use a Pythagorean triple to find x. Explain your reasoning.

21. Example 4
Notice that 24 and 26 are multiples of 2 : 24 = 2 ● 12 and 26 = 2 ● 13. Since 5, 12, 13 is a Pythagorean triple, the missing leg length x is 2 ● 5 or 10.
Answer: x = 10
Check: = 262 Pythagorean Theorem ?
676 = 676 Simplify. 

22. Your Turn:
Use a Pythagorean triple to find x.
A. 10
B. 15
C. 18
D. 24

23. More Practice
Use Pythagorean Triples to find each missing side length.
Primitive:
X=26
Primitive:
X=50
Primitive: 3-4-5
X=15