Medians, medians and more medians

Geometry Level 4

Let 0 \triangle_0 denote a triangle with area A 0 A_0 .

Triangle i \triangle_{i} if formed by taking medians of the triangle i 1 \triangle_{i-1} as side lengths.

Let the area of this triangle thus formed be A i A_i .

Find i = 0 A i \displaystyle\sum_{i=0}^{\infty}{A_i} .

3 A 0 2 \displaystyle\frac{3A_0}{2} 4 A 0 4A_0 2 A 0 2A_0 \displaystyle\infty 3 A 0 4 \displaystyle\frac{3A_0}{4}

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1 solution

A i A_i and A i 1 A_{i-1} are related by the formula A i = 3 A i 1 4 A_{i}=\displaystyle\frac{3A_{i-1}}{4}

Thus i = 0 A i = A 0 n = 0 ( 3 4 ) n = 4 A 0 \displaystyle\sum_{i=0}^{\infty}A_i=A_0\displaystyle\sum_{n=0}^{\infty}\displaystyle(\frac{3}{4})^{n}=4A_0

Proof that A i = 3 A i 1 4 A_{i}=\displaystyle\frac{3A_{i-1}}{4}

B G C G \square BGCG' is a parallelogram with sides 2 β , 2 γ , 2 β , 2 γ 2\beta, 2\gamma, 2\beta, 2\gamma , Thus area of G G C \triangle GG'C is equal to the area of B G C \triangle BGC

Now let the area of triangle with sides α , β , γ \alpha, \beta, \gamma be Δ \Delta .

Thus area of G G C \triangle GG'C which has sides 2 α , 2 β , 2 γ 2\alpha, 2\beta, 2\gamma is 4 Δ 4\Delta , Thus area of B G C \triangle BGC is 4 Δ 4\Delta ( I f s i d e s a r e s c a l e d b y a f a c t o r o f k a r e a s a r e s c a l e d b y a f a c t o r o f k 2 ) (If \ sides \ are \ scaled \ by \ a \ factor \ of \ k \ areas \ are \ scaled \ by \ a \ factor \ of \ k^{2})

By similar reasoning, we can say areas of B G C C G A & A G B \triangle BGC \ \triangle CGA \ \& \ \triangle AGB are all 4 Δ 4\Delta .

Thus area of A B C \triangle ABC is 12 Δ 12\Delta

Also, the medians have lengths 3 α , 3 β , 3 γ 3\alpha, 3\beta, 3\gamma , Thus the triangle formed by the medians will have an area of 9 Δ 9\Delta

Thus the area of triangle Δ A = 12 Δ \Delta_A=12\Delta .

And the area of the triangle formed by medians Δ M = 9 Δ \Delta_M=9\Delta .

Thus we get the relation Δ M = 3 Δ A 4 \Delta_M=\displaystyle\frac{3\Delta_A}{4} .

https://brilliant.org/problems/medians-and-sides-changed-their-roles/?ref_id=1305420

Here, you can find a similar question with a different construction. I missed it as I knew that particular ratio 3 / 4, and didn't bother going into the question.

Vishwash Kumar ΓΞΩ - 4 years, 1 month ago

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