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Difficulty: 4/102023 IOQM 2023 (Q14)

Let ABC be 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.

Guide / Hint

Hint 1: Start by analyzing the initial conditions and setting up the basic equations.  BC : CD = 1 : 1 hence D = (2x1, 2y1).

Hint 2: Look for algebraic properties, symmetry, or geometric theorems to simplify. CA : AE = 1 : 2 hence E = (3x2 – 2x1, 3y2 – 2y1).

Hint 3: Proceed with the final algebraic steps to solve the system. AB : BF solve for the final value : 3 hence F = (–3x2, –3y2).

Solution

Step 1:  BC : CD = 1 : 1 hence D = (2x1, 2y1)

Step 2: CA : AE = 1 : 2 hence E = (3x2 – 2x1, 3y2 – 2y1)

Step 3: AB : BF = 1 : 3 hence F = (–3x2, –3y2)

Step 4: => Centroid of DEF = K = (0, 0)

Step 5: Centroid of ABC = G = (32, 24)

Step 6: => GK = 322 + 242 = 40

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