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 AM (from the edge A) and BN (from the edge B) are perpendicular. Let's use vectors: c=AB, b=AC. Then AM=21(AB+AC)=21(c+b), BN=21(BA+BC)=21(2BA+AC)=21(b−2c). These vectors are perpendicular, so, their dot product is equal to 0. So, 0=(AM,BN)=41(c+b,b−2c)=41(∣b∣2−2∣c∣2−(c,b)),(∗)∣BC∣2=(c+b,c+b)=∣c∣2+∣b∣2+2(c,b). But then: (c,b)=21(BC2−∣b∣2−2∣c∣2).
Let's substitute (*): 0=∣b∣2−2∣c∣2−21(BC2−∣b∣2−∣c∣2). Let's notice, that ∣c∣2=AB2,∣b∣2=AC2.
So, we can rewrite the equation like this: 5AB2=BC2+AC2. 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 AB in this form: d2+e2. Then, (12+22)(d2+e2)(d2+e2)=((d−2e)2+(2d+e)2)(d2+e2)=(4de−d2+e2)2+(2d2+2de−2e2)2 If e=2 and d=3, the triangle with the sides AB=13, BC=19 and AC=22 satisfy the condition. So, we've found the triangle, 2 medians of which are perpendicular and the sides are integers.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: MathNet,
licensed CC-BY-4.0.
Statement and solution reproduced as published; topic and difficulty added by this site.