Very "Strict" Triangle!

Geometry Level 5

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.


The answer is 8.

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1 solution

Mark Hennings
Jun 8, 2018

If the triangle has sides a , b , c a,b,c and semiperimeter s = 1 2 ( a + b + c ) s = \tfrac12(a+ b+c) , we want a + b c , a + c b , b + c a 5 a+b-c,a+c-b,b+c-a \ge 5 , and hence we want 2 ( s a ) , 2 ( s b ) , 2 ( s c ) 5 2(s-a),2(s-b),2(s-c) \ge 5 .If we write α = 2 ( s a ) β = 2 ( s b ) γ = 2 ( s c ) \alpha \; = \; 2(s-a) \hspace{1cm} \beta \; = \; 2(s-b) \hspace{1cm} \gamma \;=\; 2(s-c) then we want α , β , γ \alpha,\beta,\gamma to be integers that are all at least 5 5 . We note that the area of the triangle is Δ = s ( s a ) ( s b ) ( s c ) = 1 4 ( α + β + γ ) α β γ \Delta \; = \; \sqrt{s(s-a)(s-b)(s-c)} \; = \; \tfrac14\sqrt{(\alpha+\beta+\gamma)\alpha\beta\gamma} while the perimeter of the triangle is 2 s = α + β + γ 2s = \alpha + \beta + \gamma . Thus we want Δ = 4 s ( α + β + γ ) α β γ = 8 ( α + β + γ ) α β γ = 64 ( α + β + γ ) \begin{aligned} \Delta & = \; 4s \\ \sqrt{(\alpha+\beta+\gamma)\alpha\beta\gamma} & = \; 8(\alpha+\beta+\gamma) \\ \alpha\beta\gamma & = \; 64(\alpha+\beta+\gamma) \end{aligned} We also require that a = 1 2 ( β + γ ) a = \tfrac12(\beta+\gamma) , b = 1 2 ( α + γ ) b = \tfrac12(\alpha+\gamma) , c = 1 2 ( α + β ) c = \tfrac12(\alpha+\beta) all to be integers, and hence we require α , β , γ \alpha,\beta,\gamma to all have the same parity. Since α β γ = 64 ( α + β + γ ) \alpha\beta\gamma = 64(\alpha+\beta+\gamma) is even, we deduce that α , β , γ \alpha,\beta,\gamma are all even, so we can write α = 2 A \alpha = 2A , β = 2 B \beta = 2B , γ = 2 C \gamma = 2C where A , B , C A,B,C are integers with A , B , C 3 A B C = 16 ( A + B + C ) A,B,C \ge 3 \hspace{2cm} ABC \; = \; 16(A+B+C) Without loss of generality we can assume that A B C A \le B \le C . Then A B C 48 C ABC \le 48C , so that A B 48 AB \le 48 . Thus we have 3 A 6 3 \le A \le 6 and A B 48 A A \le B \le \tfrac{48}{A} , so there are only a finitely many possible values for A , B A,B . Checking these, we obtain the following 8 8 possible triples for ( A , B , C ) (A,B,C) : ( 3 , 6 , 72 ) ( 3 , 7 , 32 ) ( 3 , 8 , 22 ) ( 3 , 12 , 12 ) ( 4 , 5 , 36 ) ( 4 , 6 , 20 ) ( 4 , 8 , 12 ) ( 6 , 7 , 8 ) \begin{array}{cccc} (3,6,72) & (3,7,32) & (3,8,22) & (3,12,12) \\ (4,5,36) & (4,6,20) & (4,8,12) & (6,7,8) \end{array} and hence the following 8 \boxed{8} triples for the triangle sides ( a , b , c ) (a,b,c) : ( 78 , 75 , 9 ) ( 39 , 35 , 10 ) ( 30 , 25 , 11 ) ( 24 , 15 , 15 ) ( 41 , 40 , 9 ) ( 26 , 24 , 10 ) ( 20 , 16 , 12 ) ( 15 , 14 , 13 ) \begin{array}{cccc} (78,75,9) & (39,35,10) & (30,25,11) & (24,15,15) \\ (41,40,9) & (26,24,10) & (20,16,12) & (15,14,13) \end{array}

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