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Geometry Difficulty 7.4 National olympiad, round 2 Find the answer

Let ABCA B C be a triangle in the xyx y plane, where BB is at the origin (0,0)(0,0). Let BCB C be produced to DD such that BC:CD=1:1,CAB C: C D=1: 1, C A be produced to EE such that CA:AE=1:2C A: A E=1: 2 and ABA B be produced to FF such that AB:BF=1:3A B: B F=1: 3. Let G(32,24)G(32,24) be the centroid of the triangle ABCA B C and KK be the centroid of the triangle DEFD E F. Find the length GKG K.

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

Solution

1. **Determine the coordinates of points AA, BB, and CC**:
- Let B=(0,0)B = (0,0).
- Let C=(x,0)C = (x,0) for some xx.
- Let A=(a,b)A = (a,b).

2. **Find the coordinates of points DD, EE, and FF**:
- Since BC:CD=1:1BC:CD = 1:1, DD is at (2x,0)(2x,0).
- Since CA:AE=1:2CA:AE = 1:2, EE is at (a2,b2)(-\frac{a}{2}, -\frac{b}{2}).
- Since AB:BF=1:3AB:BF = 1:3, FF is at (3a,3b)(3a, 3b).

3. **Find the centroid GG of triangle ABCABC**:
- The centroid GG of a triangle with vertices (x1,y1)(x_1, y_1), (x2,y2)(x_2, y_2), (x3,y3)(x_3, y_3) is given by:
G=(x1+x2+x33,y1+y2+y33) G = \left( \frac{x_1 + x_2 + x_3}{3}, \frac{y_1 + y_2 + y_3}{3} \right)
- For triangle ABCABC:
G=(0+x+a3,0+0+b3) G = \left( \frac{0 + x + a}{3}, \frac{0 + 0 + b}{3} \right)
- Given G=(32,24)G = (32, 24), we have:
x+a3=32andb3=24 \frac{x + a}{3} = 32 \quad \text{and} \quad \frac{b}{3} = 24
x+a=96andb=72 x + a = 96 \quad \text{and} \quad b = 72

4. **Find the centroid KK of triangle DEFDEF**:
- For triangle DEFDEF:
K=(2xa2+3a3,0b2+3b3) K = \left( \frac{2x - \frac{a}{2} + 3a}{3}, \frac{0 - \frac{b}{2} + 3b}{3} \right)
K=(2x+5a23,5b23) K = \left( \frac{2x + \frac{5a}{2}}{3}, \frac{\frac{5b}{2}}{3} \right)
K=(4x+5a6,5b6) K = \left( \frac{4x + 5a}{6}, \frac{5b}{6} \right)

5. **Substitute the values of aa and bb**:
- From step 3, b=72b = 72:
K=(4x+5a6,5726) K = \left( \frac{4x + 5a}{6}, \frac{5 \cdot 72}{6} \right)
K=(4x+5a6,60) K = \left( \frac{4x + 5a}{6}, 60 \right)

6. **Determine the coordinates of KK**:
- From step 3, x+a=96x + a = 96:
K=(4x+5(96x)6,60) K = \left( \frac{4x + 5(96 - x)}{6}, 60 \right)
K=(4x+4805x6,60) K = \left( \frac{4x + 480 - 5x}{6}, 60 \right)
K=(480x6,60) K = \left( \frac{480 - x}{6}, 60 \right)

7. **Find the distance GKGK**:
- G=(32,24)G = (32, 24) and K=(480x6,60)K = \left( \frac{480 - x}{6}, 60 \right):
GK=(32480x6)2+(2460)2 GK = \sqrt{\left( 32 - \frac{480 - x}{6} \right)^2 + (24 - 60)^2}
GK=(192480+x6)2+(36)2 GK = \sqrt{\left( \frac{192 - 480 + x}{6} \right)^2 + (-36)^2}
GK=(288+x6)2+1296 GK = \sqrt{\left( \frac{-288 + x}{6} \right)^2 + 1296}
GK=(x2886)2+1296 GK = \sqrt{\left( \frac{x - 288}{6} \right)^2 + 1296}
GK=(x288)236+1296 GK = \sqrt{\frac{(x - 288)^2}{36} + 1296}
GK=(x288)2+4665636 GK = \sqrt{\frac{(x - 288)^2 + 46656}{36}}
GK=(x288)2+466566 GK = \frac{\sqrt{(x - 288)^2 + 46656}}{6}

8. Simplify the expression:
- Since x=96ax = 96 - a and a=96xa = 96 - x, we can substitute x=96x = 96:
GK=(96288)2+466566 GK = \frac{\sqrt{(96 - 288)^2 + 46656}}{6}
GK=(192)2+466566 GK = \frac{\sqrt{(-192)^2 + 46656}}{6}
GK=36864+466566 GK = \frac{\sqrt{36864 + 46656}}{6}
GK=835206 GK = \frac{\sqrt{83520}}{6}
GK=2886 GK = \frac{288}{6}
GK=48 GK = 48

The final answer is 48\boxed{48}.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.