How many different triangles with integer side lengths are there such that the sum of the lengths of any 2 sides is at least 5 units more than the length of the third side, and that the area is numerically twice the perimeter?
Note: two triangles are regarded to be the same if they are congruent.
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
If the triangle has sides a , b , c and semiperimeter s = 2 1 ( a + b + c ) , we want a + b − c , a + c − b , b + c − a ≥ 5 , and hence we want 2 ( s − a ) , 2 ( s − b ) , 2 ( s − c ) ≥ 5 .If we write α = 2 ( s − a ) β = 2 ( s − b ) γ = 2 ( s − c ) then we want α , β , γ to be integers that are all at least 5 . We note that the area of the triangle is Δ = s ( s − a ) ( s − b ) ( s − c ) = 4 1 ( α + β + γ ) α β γ while the perimeter of the triangle is 2 s = α + β + γ . Thus we want Δ ( α + β + γ ) α β γ α β γ = 4 s = 8 ( α + β + γ ) = 6 4 ( α + β + γ ) We also require that a = 2 1 ( β + γ ) , b = 2 1 ( α + γ ) , c = 2 1 ( α + β ) all to be integers, and hence we require α , β , γ to all have the same parity. Since α β γ = 6 4 ( α + β + γ ) is even, we deduce that α , β , γ are all even, so we can write α = 2 A , β = 2 B , γ = 2 C where A , B , C are integers with A , B , C ≥ 3 A B C = 1 6 ( A + B + C ) Without loss of generality we can assume that A ≤ B ≤ C . Then A B C ≤ 4 8 C , so that A B ≤ 4 8 . Thus we have 3 ≤ A ≤ 6 and A ≤ B ≤ A 4 8 , so there are only a finitely many possible values for A , B . Checking these, we obtain the following 8 possible triples for ( A , B , C ) : ( 3 , 6 , 7 2 ) ( 4 , 5 , 3 6 ) ( 3 , 7 , 3 2 ) ( 4 , 6 , 2 0 ) ( 3 , 8 , 2 2 ) ( 4 , 8 , 1 2 ) ( 3 , 1 2 , 1 2 ) ( 6 , 7 , 8 ) and hence the following 8 triples for the triangle sides ( a , b , c ) : ( 7 8 , 7 5 , 9 ) ( 4 1 , 4 0 , 9 ) ( 3 9 , 3 5 , 1 0 ) ( 2 6 , 2 4 , 1 0 ) ( 3 0 , 2 5 , 1 1 ) ( 2 0 , 1 6 , 1 2 ) ( 2 4 , 1 5 , 1 5 ) ( 1 5 , 1 4 , 1 3 )