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Geometry Difficulty 5.9 AIME, harder Prove it Ukraine

Does the triangle, sides of which can be expressed as a positive integer in centimeters and 2 of its medians are perpendicular, exist?

Solution

Let's find the condition on the sides of the triangle, under which the medians are perpendicular. Without loss of generality let's consider that medians AMAM (from the edge AA) and BNBN (from the edge BB) are perpendicular. Let's use vectors: c=AB\vec{c} = \overrightarrow{AB}, b=AC\vec{b} = \overrightarrow{AC}. Then AM=12(AB+AC)=12(c+b)\overrightarrow{AM} = \frac{1}{2}(\overrightarrow{AB} + \overrightarrow{AC}) = \frac{1}{2}(\vec{c} + \vec{b}), BN=12(BA+BC)=12(2BA+AC)=12(b2c)\overrightarrow{BN} = \frac{1}{2}(\overrightarrow{BA} + \overrightarrow{BC}) = \frac{1}{2}(2\overrightarrow{BA} + \overrightarrow{AC}) = \frac{1}{2}(\vec{b} - 2\vec{c}). These vectors are perpendicular, so, their dot product is equal to 00. So,
0=(AM,BN)=14(c+b,b2c)=14(b22c2(c,b)),()BC2=(c+b,c+b)=c2+b2+2(c,b). But then: (c,b)=12(BC2b22c2). \begin{gathered} 0 = (\overrightarrow{AM}, \overrightarrow{BN}) = \frac{1}{4}(\vec{c} + \vec{b}, \vec{b} - 2\vec{c}) = \frac{1}{4}(|\vec{b}|^2 - 2|\vec{c}|^2 - (\vec{c}, \vec{b})), \quad (*) \\ |\overrightarrow{BC}|^2 = (\vec{c} + \vec{b}, \vec{c} + \vec{b}) = |\vec{c}|^2 + |\vec{b}|^2 + 2(\vec{c}, \vec{b}). \text{ But then: } (\vec{c}, \vec{b}) = \frac{1}{2}(BC^2 - |\vec{b}|^2 - 2|\vec{c}|^2). \end{gathered}

Let's substitute (*): 0=b22c212(BC2b2c2)0 = |\vec{b}|^2 - 2|\vec{c}|^2 - \frac{1}{2}(BC^2 - |\vec{b}|^2 - |\vec{c}|^2).
Let's notice, that c2=AB2,b2=AC2|\vec{c}|^2 = AB^2, |\vec{b}|^2 = AC^2.

So, we can rewrite the equation like this: 5AB2=BC2+AC25AB^2 = BC^2 + AC^2. Let's notice, that this condition is necessary and sufficient for medians to be perpendicular. That's why the only thing that remains is to check if this equation has the solution in integers, that is true for triangle inequality. Let's find ABAB in this form: d2+e2d^2 + e^2. Then,
(12+22)(d2+e2)(d2+e2)=((d2e)2+(2d+e)2)(d2+e2)=(4ded2+e2)2+(2d2+2de2e2)2 (1^2 + 2^2)(d^2 + e^2)(d^2 + e^2) = ((d-2e)^2 + (2d+e)^2)(d^2 + e^2) \\ = (4de - d^2 + e^2)^2 + (2d^2 + 2de - 2e^2)^2
If e=2e=2 and d=3d=3, the triangle with the sides AB=13AB=13, BC=19BC=19 and AC=22AC=22 satisfy the condition. So, we've found the triangle, 2 medians of which are perpendicular and the sides are integers.

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