GeometryDifficulty 7.4National olympiad, round 2Find the answer
Let ABC be a triangle in the xy plane, where B is at the origin (0,0). Let BC be produced to D such that BC:CD=1:1,CA be produced to E such that CA:AE=1:2 and AB be produced to F such that AB:BF=1:3. Let G(32,24) be the centroid of the triangle ABC and K be the centroid of the triangle DEF. Find the length GK.
A number or a short expression. Spacing and $ signs are ignored.
Solution
1. **Determine the coordinates of points A, B, and C**: - Let B=(0,0). - Let C=(x,0) for some x. - Let A=(a,b).
2. **Find the coordinates of points D, E, and F**: - Since BC:CD=1:1, D is at (2x,0). - Since CA:AE=1:2, E is at (−2a,−2b). - Since AB:BF=1:3, F is at (3a,3b).
3. **Find the centroid G of triangle ABC**: - The centroid G of a triangle with vertices (x1,y1), (x2,y2), (x3,y3) is given by: G=(3x1+x2+x3,3y1+y2+y3) - For triangle ABC: G=(30+x+a,30+0+b) - Given G=(32,24), we have: 3x+a=32and3b=24 x+a=96andb=72
4. **Find the centroid K of triangle DEF**: - For triangle DEF: K=(32x−2a+3a,30−2b+3b) K=(32x+25a,325b) K=(64x+5a,65b)
5. **Substitute the values of a and b**: - From step 3, b=72: K=(64x+5a,65⋅72) K=(64x+5a,60)
6. **Determine the coordinates of K**: - From step 3, x+a=96: K=(64x+5(96−x),60) K=(64x+480−5x,60) K=(6480−x,60)
7. **Find the distance GK**: - G=(32,24) and K=(6480−x,60): GK=(32−6480−x)2+(24−60)2 GK=(6192−480+x)2+(−36)2 GK=(6−288+x)2+1296 GK=(6x−288)2+1296 GK=36(x−288)2+1296 GK=36(x−288)2+46656 GK=6(x−288)2+46656
8. Simplify the expression: - Since x=96−a and a=96−x, we can substitute x=96: GK=6(96−288)2+46656 GK=6(−192)2+46656 GK=636864+46656 GK=683520 GK=6288 GK=48
The final answer is 48.
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: NuminaMath-1.5,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.