Difference between revisions of "Pythagorean Theorem"
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=== Introductory === | === Introductory === | ||
* [[2006_AIME_I_Problems/Problem_1 | 2006 AIME I Problem 1]] | * [[2006_AIME_I_Problems/Problem_1 | 2006 AIME I Problem 1]] | ||
+ | * [[2007 AMC 12A Problems/Problem 10 | 2007 AMC 12A Problem 10]] | ||
== External links == | == External links == | ||
*[http://www.cut-the-knot.org/pythagoras/index.shtml 75 proofs of the Pythagorean Theorem] | *[http://www.cut-the-knot.org/pythagoras/index.shtml 75 proofs of the Pythagorean Theorem] |
Revision as of 07:12, 8 October 2007
The Pythagorean Theorem states that for a right triangle with legs of length and
and hypotenuse of length
we have the relationship
. This theorem has been know since antiquity and is a classic to prove; hundreds of proofs have been published and many can be demonstrated entirely visually. The Pythagorean Theorem is one of the most frequently used theorems in geometry, and is one of the many tools in a good geometer's arsenal. A very large number of geometry problems can be solved by building right triangles and applying the Pythagorean Theorem.
This is generalized by the Pythagorean Inequality and the Law of Cosines.
Contents
[hide]Proofs
In these proofs, we will let be any right triangle with a right angle at
.
Proof 1
We use to denote the area of triangle
.
Let be the perpendicular to side
from
.

Since are similar right triangles, and the areas of similar triangles are proportionate to the squares of corresponding side lengths,
.
But since triangle is composed of triangles
and
,
, so
. ∎
Proof 2
Consider a circle with center
and radius
. Since
and
are perpendicular,
is tangent to
. Let the line
meet
at
and
, as shown in the diagram:

Evidently, and
. By considering the power of point
with respect to
, we see
. ∎
Common Pythagorean Triples
A Pythagorean Triple is a set of 3 positive integers such that , i.e. the 3 numbers can be the lengths of the sides of a right triangle. Among these, the Primitive Pythagorean Triples, those in which the three numbers have no common divisor, are most interesting. A few of them are:
3-4-5
5-12-13
7-24-25
8-15-17
9-40-41