Question 15

Equilateral triangle $$\triangle ABC$$ has centroid G. $$A_1$$, $$B_1$$, $$C_1$$ are points on $$\overline{AG}$$, $$\overline{BG}$$, $$\overline{CG}$$ such that $$\overleftrightarrow{A_1B_1}$$, $$\overleftrightarrow{B_1C_1}$$, $$\overleftrightarrow{C_1A_1}$$ are respectively parallel to $$\overleftrightarrow{AB}$$, $$\overleftrightarrow{BC}$$, $$\overleftrightarrow{CA}$$. If the distance between $$\overleftrightarrow{BC}$$ and $$\overleftrightarrow{B_1C_1}$$ is one-sixth of the altitude of $$\triangle ABC$$, determine the ratio of areas $$\frac{\triangle A_1B_1C_1}{\triangle ABC}$$.


Correct Answer: 0.25

Solution

$$\triangle A_1B_1C_1$$ is the image of $$\triangle ABC$$ under an enlargement centred at G. If $$h$$ is the altitude, G is $$\frac{h}{3}$$ from BC, and $$B_1C_1$$ lies between G and BC at $$\frac{h}{6}$$ from BC, i.e. $$\frac{h}{6}$$ from G. The scale factor is $$\frac{h/6}{h/3} = \frac{1}{2}$$, so the ratio of areas is $$\frac{1}{4} = 0.25$$.

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