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Geometry Difficulty 7.6 National olympiad, round 2 Find the answer

A line in the plane of a triangle TT is called an \emph{equalizer} if it divides TT into two regions having equal area and equal perimeter. Find positive integers a>b>ca>b>c, with aa as small as possible, such that there exists a triangle with side lengths a,b,ca, b, c that has exactly two distinct equalizers.

A number or a short expression. Spacing and $ signs are ignored.

Solution

The desired integers are (a,b,c)=(9,8,7)(a,b,c) = (9,8,7). Suppose we have a triangle T=ABCT = \triangle ABC with BC=aBC=a, CA=bCA=b, AB=cAB=c and a>b>ca>b>c. Say that a line is an \textit{area equalizer} if it divides TT into two regions of equal area. A line intersecting TT must intersect two of the three sides of TT. First consider a line intersecting the segments ABAB at XX and BCBC at YY, and let BX=xBX=x, BY=yBY=y. This line is an area equalizer if and only if xysinB=2area(XBY)=area(ABC)=12acsinBxy\sin B = 2\operatorname{area}(\triangle XBY) = \operatorname{area}(\triangle ABC) = \frac{1}{2}ac\sin B, that is, 2xy=ac2xy=ac. Since xcx \leq c and yay \leq a, the area equalizers correspond to values of x,yx,y with xy=ac/2xy=ac/2 and x[c/2,c]x \in [c/2,c]. Such an area equalizer is also an equalizer if and only if p/2=x+yp/2=x+y, where p=a+b+cp=a+b+c is the perimeter of TT. If we write f(x)=x+ac/(2x)f(x) = x+ac/(2x), then we want to solve f(x)=p/2f(x) = p/2 for x[c/2,c]x \in [c/2,c]. Now note that ff is convex, f(c/2)=a+c/2>p/2f(c/2) = a+c/2 > p/2, and f(c)=a/2+c<p/2f(c) = a/2+c < p/2; it follows that there is exactly one solution to f(x)=p/2f(x)=p/2 in [c/2,c][c/2,c]. Similarly, for equalizers intersecting TT on the sides ABAB and ACAC, we want to solve g(x)=p/2g(x) = p/2 where g(x)=x+bc/(2x)g(x) = x+bc/(2x) and x[c/2,c]x \in [c/2,c]; since gg is convex and g(c/2)<p/2g(c/2)<p/2, g(c)<p/2g(c) < p/2, there are no such solutions. It follows that if TT has exactly two equalizers, then it must have exactly one equalizer intersecting TT on the sides ACAC and BCBC. Here we want to solve h(x)=p/2h(x) = p/2 where h(x)=x+ab/(2x)h(x) = x+ab/(2x) and x[a/2,a]x \in [a/2,a]. Now hh is convex and h(a/2)>p/2h(a/2) > p/2, h(a)>p/2h(a) > p/2; thus h(x)=p/2h(x) = p/2 has exactly one solution x[a/2,a]x \in [a/2,a] if and only if there is x0[a/2,a]x_0 \in [a/2,a] with h(x0)=0h'(x_0) = 0 and h(x0)=p/2h(x_0) = p/2. The first condition implies x0=ab/2x_0 = \sqrt{ab/2}, and then the second condition gives 8ab=p28ab = p^2. Note that ab/2\sqrt{ab/2} is in [a/2,a][a/2,a] since a>ba>b and a<b+c<2ba<b+c<2b. We conclude that TT has two equalizers if and only if 8ab=(a+b+c)28ab=(a+b+c)^2. Note that (a,b,c)=(9,8,7)(a,b,c) = (9,8,7) works. We claim that this is the only possibility when a>b>ca>b>c are integers and a9a \leq 9. Indeed, the only integers (a,b)(a,b) such that 2b<a92 \leq b < a \leq 9 and 8ab8ab is a perfect square are (a,b)=(4,2)(a,b) = (4,2), (6,3)(6,3), (8,4)(8,4), (9,2)(9,2), and (9,8)(9,8), and the first four possibilities do not produce triangles since they do not satisfy a<2ba<2b. This gives the claimed result.

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