In a △ A B C with integral sides such that A B × A C = 5 6 .Find the number of such triangles possible.
Note: A B = 7 , B C = 4 , A C = 8 and A B = 8 , B C = 4 , A C = 7 represent 2 different triangles.
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You could have used basic triangle Inequality
This is what's called true maths. The Solution is astonishing.
The possible values of A B , A C are ( 1 , 5 6 ) , ( 2 , 2 8 ) , ( 4 , 1 4 ) , ( 7 , 8 ) , ( 8 , 7 ) , ( 1 4 , 4 ) , ( 2 8 , 2 ) , ( 5 6 , 1 )
Case 1: 1 + 5 6 > x g i v e s x < 5 7
x + 1 > 5 6 g i v e s x > 5 5
So the sides are ( 5 6 , 1 , 5 6 )
Case 2: 2 + 2 8 > x gives x < 3 0
x + 2 > 2 8 gives x > 2 6
So The sides are ( 2 8 , 2 , 2 7 ) , ( 2 8 , 2 , 2 8 ) , ( 2 8 , 2 , 2 9 )
Case 3: 4 + 1 4 > x gives x < 1 8
x + 4 > 1 4 gives x > 1 0
Just as above there are 7 values satisfying 1 0 < x < 1 8
Case 4: 8 + 7 > x gives x < 1 5
x + 7 > 8 gives x > 1
There are 1 3 values for 1 < x < 1 5
From the above cases 24 triplets of sides are possible,but the values of sides A B and A C can be exchanged, so there are 2 4 ∗ 2 = 4 8 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. :(
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Here the product of A B and A C is fixed,so we can only exchange the values between them,the value of B C cannot be exchanged with A B or A C :)
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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 − 1 1 2 cos 0 ∘ < B C 2 < A B 2 + A C 2 − 1 1 2 cos 1 8 0 ∘ A B 2 + A C 2 − 1 1 2 < B C < A B 2 + A C 2 + 1 1 2
For the 8 possible cases of A B and A C , we have:
⎩ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎧ ( A B , A C ) = ( 1 , 5 6 ) ; ( 5 6 , 1 ) ( A B , A C ) = ( 2 , 2 8 ) ; ( 2 8 , 2 ) ( A B , A C ) = ( 4 , 1 4 ) ; ( 1 4 , 4 ) ( A B , A C ) = ( 7 , 8 ) ; ( 8 , 7 ) ⇒ 5 5 < B C < 5 7 ⇒ 2 6 < B C < 3 0 ⇒ 1 0 < B C < 1 8 ⇒ 1 < B C < 1 5 ⇒ { acceptable B C = 5 6 acceptable triangles = 2 ⇒ { acceptable B C = 2 7 , 2 8 , 2 9 acceptable triangles = 6 ⇒ { acceptable B C = 1 1 , 1 2 , 1 3 , . . . 1 7 acceptable triangles = 1 4 ⇒ { acceptable B C = 2 , 3 , 4 , . . . 1 4 acceptable triangles = 2 6
⇒ The total number of acceptable triangles is = 2 + 6 + 1 4 + 2 6 = 4 8 .