Triangle's possibility

Geometry Level 4

In a A B C \displaystyle\triangle ABC with integral sides such that A B × A C = 56 AB×AC=56 .Find the number of such triangles possible.

Note: A B = 7 , B C = 4 , A C = 8 AB=7,BC=4,AC=8 and A B = 8 , B C = 4 , A C = 7 AB=8,BC=4,AC=7 represent 2 different triangles.


Problem is original.


The answer is 48.

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2 solutions

Chew-Seong Cheong
Nov 22, 2015

Using Cosine Rule, we have:

B C 2 = A B 2 + A C 2 2 ˙ A B ˙ A C ˙ cos θ A B 2 + A C 2 112 cos 0 < B C 2 < A B 2 + A C 2 112 cos 18 0 A B 2 + A C 2 112 < B C < A B 2 + A C 2 + 112 BC^2 = AB^2 + AC^2 - 2\dot{}AB\dot{}AC\dot{}\cos\theta \\ \Rightarrow AB^2 + AC^2 - 112 \cos 0^\circ < BC^2 < AB^2 + AC^2 - 112 \cos 180^\circ \\ \sqrt{AB^2 + AC^2 - 112} < BC < \sqrt{AB^2 + AC^2 + 112}

For the 8 8 possible cases of A B AB and A C AC , we have:

{ ( A B , A C ) = ( 1 , 56 ) ; ( 56 , 1 ) 55 < B C < 57 { acceptable B C = 56 acceptable triangles = 2 ( A B , A C ) = ( 2 , 28 ) ; ( 28 , 2 ) 26 < B C < 30 { acceptable B C = 27 , 28 , 29 acceptable triangles = 6 ( A B , A C ) = ( 4 , 14 ) ; ( 14 , 4 ) 10 < B C < 18 { acceptable B C = 11 , 12 , 13 , . . . 17 acceptable triangles = 14 ( A B , A C ) = ( 7 , 8 ) ; ( 8 , 7 ) 1 < B C < 15 { acceptable B C = 2 , 3 , 4 , . . . 14 acceptable triangles = 26 \begin{cases} (AB,AC) = (1,56); (56,1) & \Rightarrow 55 < BC < 57 & \Rightarrow \begin{cases} \text{acceptable } BC = 56 \\ \text{acceptable triangles} = 2 \end{cases} \\ (AB,AC) = (2,28); (28,2) & \Rightarrow 26 < BC < 30 & \Rightarrow \begin{cases} \text{acceptable } BC = 27,28,29 \\ \text{acceptable triangles} = 6 \end{cases} \\ (AB,AC) = (4,14); (14,4) & \Rightarrow 10 < BC < 18 & \Rightarrow \begin{cases} \text{acceptable } BC = 11,12,13,...17 \\ \text{acceptable triangles} = 14 \end{cases} \\ (AB,AC) = (7,8); (8,7) & \Rightarrow 1 < BC < 15 & \Rightarrow \begin{cases} \text{acceptable } BC = 2,3,4,...14 \\ \text{acceptable triangles} = 26 \end{cases} \end{cases}

\Rightarrow The total number of acceptable triangles is = 2 + 6 + 14 + 26 = 48 =2+6+14+26 = \boxed{48} .

You could have used basic triangle Inequality

Department 8 - 5 years, 6 months ago

This is what's called true maths. The Solution is astonishing.

Vishal Yadav - 5 years, 6 months ago
Siddharth Singh
Nov 21, 2015

The possible values of A B , A C AB ,AC are ( 1 , 56 ) , ( 2 , 28 ) , ( 4 , 14 ) , ( 7 , 8 ) , ( 8 , 7 ) , ( 14 , 4 ) , ( 28 , 2 ) , ( 56 , 1 ) (1,56),(2,28),(4,14),(7,8),(8,7),(14,4),(28,2),(56,1)

Case 1: 1 + 56 > x g i v e s x < 57 1+56>x gives x<57

x + 1 > 56 g i v e s x > 55 x+1>56 gives x>55

So the sides are ( 56 , 1 , 56 ) (56,1,56)

Case 2: 2 + 28 > x 2+28>x gives x < 30 x<30

x + 2 > 28 x+2>28 gives x > 26 x>26

So The sides are ( 28 , 2 , 27 ) , ( 28 , 2 , 28 ) , ( 28 , 2 , 29 ) (28,2,27),(28,2,28),(28,2,29)

Case 3: 4 + 14 > x 4+14>x gives x < 18 x<18

x + 4 > 14 x+4>14 gives x > 10 x>10

Just as above there are 7 7 values satisfying 10 < x < 18 10<x<18

Case 4: 8 + 7 > x 8+7>x gives x < 15 x<15

x + 7 > 8 x+7>8 gives x > 1 x>1

There are 13 13 values for 1 < x < 15 1<x<15

From the above cases 24 triplets of sides are possible,but the values of sides A B AB and A C AC can be exchanged, so there are 24 2 = 48 24*2=\boxed{48} such triplets.

If changing AB and AC makes a different triangle then by exchanging the other other side will also give a new triangle. Thus if two sides are equal there are 3 different triangles and if all sides are different we get 6 different triangles. Doing this way the answer is coming 264. :(

Samarth Agarwal - 5 years, 6 months ago

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Here the product of A B AB and A C AC is fixed,so we can only exchange the values between them,the value of B C BC cannot be exchanged with A B AB or A C AC :)

Siddharth Singh - 5 years, 6 months ago

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