Pythagorean Theorem: Why Use the Converse?
Date: 07/15/2003 at 20:46:30 From: Akima Subject: The Converse of the Pythagorean Theorem Dear Dr. Math - I don't really understand the concept of the 'converse' of the Pythagorean Theorem. Why use the converse? What is it useful for? My main problem when using it is to determine if the measures given form a triangle, and if it is a triangle, whether or not it is a right tringle. For example: 5, 7, 9. I don't understand what my book is asking. Another concept: The measures given form a triangle. Classify each tringle as right, acute, or obtuse. For example: 8, 8, 9. - Akima
Date: 07/15/2003 at 23:29:07 From: Doctor Peterson Subject: Re: The Converse of the Pythagorean Theorem Hi, Akima. You can start with our FAQ on the Pythagorean theorem itself: Pythagorean theorem http://mathforum.org/dr.math/faq/faq.pythagorean.html The theorem says that IF a triangle is a right triangle, THEN the sum of the squares of the two shorter sides equals the square of the longest side (the hypotenuse). The converse says the opposite: IF the sum of the squares of the two shorter sides equals the square of the longest side, THEN the triangle is a right triangle (and the angle opposite the longest side is the right angle). So let's consider a triangle with sides 5, 6, and 7 (to use one different from your problems). First we want to know if it IS a triangle in the first place - can you use these lengths to make one? For that, you use the Triangle Inequality, which says that the sum of any two sides has to be greater than the third side. Imagine having three sticks of these lengths. Connect the two shorter sticks with a flexible hinge, and bend them at different angles. The distance between the far ends, where the third stick has to go, can vary from the difference of the two lengths, 6-5=1: 5 +-----------+ <-- third side goes here, length 1 +--------------+ 6 to the sum of the lengths, 5 + 6 = 11: +-----------+-------------+ <-- third side goes all the way across 5 6 Since the longest side is less than 11, we CAN make a triangle. Now we want to see whether the triangle is right, acute, or obtuse. That is, is the largest angle a right angle, or less or more than that? For that, we use the converse of the Pythagorean theorem. Add the sums of the smaller lengths: 5^2 + 6^2 = 25 + 36 = 61 Then square the longest side: 7^2 = 49 These are NOT equal, so it is not a right triangle. If it HAD been a right triangle, it would be + | \ | \ sqrt(61) 5| \ | \ | \ +--------------+ 6 Another theorem tells us that because the longest side is SMALLER than it has to be for a right triangle, the angle opposite it is SMALLER than a right angle. So this is an acute triangle; even its largest angle is acute, so all of them are. If the square of the longest side had been larger than the sum of the squares of the others, then it would be "too big" for a right angle, and the angle opposite it would be obtuse. Here is another place to read more about this: Converse of the Pythagorean Theorem http://mathforum.org/library/drmath/view/62215.html - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994-2015 The Math Forum