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PYTHAGOREAN THEOREM. Pythagorean Theorem. Over 2,500 years ago, a Greek mathematician named Pythagoras developed a proof that the relationship between the hypotenuse and the legs is true for all right triangles .

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Pythagorean Theorem

Over 2,500 years ago, a Greek mathematician named Pythagoras developed a proof that the relationship between the hypotenuse and the legs is true for all right triangles.

In any right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the legs."

This relationship can be stated as:

and is known as the Pythagorean Theorem

a, b are legs.c is the hypotenuse (across from the right angle).

There are certain sets of numbers that have a very special property.  Not only do these numbers satisfy the Pythagorean Theorem, but any multiples of these numbers also satisfy the Pythagorean Theorem

For example:  the numbers 3, 4, and 5 satisfy the Pythagorean Theorem.  If you multiply all three numbers by 2  (6, 8, and 10), these new numbers ALSO satisfy the Pythagorean theorem.  


The Pythagorean Theorem

  • The Pythagorean Theoremis one of Euclidean Geometry's most beautiful theorems. It is simple, yet obscure, and is used continuously in mathematics and physics. In short, it is really cool.
  • This first method is one of the ways the Pythagoreans would have proved the theorem. Unfortunately, it lacks glamour. In the following picture let ABC be a right triangle and BD be a segment drawn perpendicular to AC.
  • Since the triangles are similar, the sides must be of proportional lengths.
    • AB/AD=AC/AB, or AB x AB = AD x AC
    • BC/CD=AC/BC, or BC x BC= AC x CD
    • Then, adding the two together, BC^2 + AB^2 = (AD + DC) x AC= AC^2

If we think about a right triangle we know of course that one of the angles is a right angle. We also know that the other two angles are acute angles (why?). In fact we know that the other two angles are complementary angles. Therefore there is a relationship between the sizes of the angles that the two acute angles have measures that add up to ninety degrees.

What about sides? Is there a relationship between the sides of a right triangle? We know from previous lessons that if we have the lengths of just two of the sides we can construct the triangle so it is enough to know the lengths of two sides to determine the length of the third side. We shall now try to figure out the relationship. We shall, to make it easy to communicate assume that the length of the hypotenuse is c units and that the two legs are of length a and b units.

So according to the Pythagorean Theorem, the area of square A, plus the area of square B should equal the area of square C.

The special sets of numbers that possess this property are called 

Pythagorean Triples.

The most common Pythagorean Triples are:

3, 4, 5

5, 12, 13

8, 15, 17


Pythagorean Theorem Given:- ABC is a right angle Triangle. angle B =900R.T.P:- AC2 = AB2 +BC2Construction:- To draw BD  AC .





Proof:- In Triangles ADB and ABC

AngleA=Angle A (common)

Angle ADB=Angle ABC (each 900 )

ADB ~ ABC ( A.A corollary )

So that AD/AB=AB/AC (In similar triangles corresponding sides are proportional )

AB2 = AD X AC _________(1)

Similarly BC2 = DCXAC _________(2)

Adding (1)&(2) we get


= AC (AD +DC)

= AC . AC


There fore AB2 +BC2 =AC2



Example 1. Find the length of AC.






Solution :

AC2 = 122 + 162 (Pythagoras’ Theorem)

AC2 = 144 + 256

AC2 = 400

AC = 20


d2 = 102 + 242 (Pythagoras’ Theorem)




Example 2. Find the length of diagonal d .



Application of Pythagoras’ Theorem

1.A car travels 16 km from east to west. Then it turns left and travels a further 12 km.Find the displacement between the starting point and the destination point of the car.






16 km



12 km


Solution :

In the figure,

AB = 16

BC = 12

AC2 = AB2 + BC2 (Pythagoras’ Theorem)

AC2 = 162 + 122

AC2 = 400

AC = 20

The displacement between the starting point and the destination point of the car is 20 km


13 m

5 m

2. The height of a tree is 5 m. The distance between

the top of it and the tip of its shadow is 13 m.

Find the length of the shadow L.


132 = 52 + L2 (Pythagoras’ Theorem)

L2 = 132 - 52

L2 = 144

L = 12



. 1.Find the length of the hypotenuse for right triangles with legs, and sketch the trianglesa).3 and 4?b).5 and 12?

c).5.2 and 10.5?

2.Find the lengths of the other leg of right triangles if one leg =6 and the hypotenuse =2,8.3 and 7 in each case.

3. A right triangle with legs equal to 5cm and 12cm. What is the length of the hypotenuse? .

4. In your own words, explain what the Pythagorean Theorem states.


Text book of class XSubject:MathematicsSearch

Prepared by S.Jaya Prada