Compare Medians against Side Comparison

Geometry Level 3

In Δ A B C \Delta ABC

Sides: A B = c , C A = b , B C = a ; AB=c, CA=b, BC=a;

Medians: A D = d , B E = e , C F = f . AD=d, BE=e, CF=f.

Given a > b > c a>b>c , which of the medians is the largest?

d d e e f f

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2 solutions

Solution is obvious naming M as the point where the three medians meet. The three triangles AMB, BMC, CMA have same area therefore the smallest side base c AB in our case has to have the largest median CF.

The picture below will help to visualize the proof, just comparing areas of colored paralellograms.

Why? Median doesn't mean the height of the small triangle.

Chan Tin Ping - 3 years, 5 months ago

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therefore the shorter the median the larger the sine and so the side opposed to it.

Mariano PerezdelaCruz - 3 years, 5 months ago

From comparison among the equal areas of triangle AMB = 2/9 d e sin(AMB) ; to area AMC = 2/9 d f sin(AMC) and to the area CMA = 2/9 f e sin(CMA). since d < e < f therefore

Mariano PerezdelaCruz - 3 years, 5 months ago

I will apply the Apollonius' Theorem .


By applying Apollonius' Theorem , we can have─

c 2 + b 2 = 2 ( d 2 + ( a 2 ) 2 ) c^2 + b^2 = 2 ( d^2 + (\frac{a}{2})^2)

c 2 + b 2 = 2 ( d 2 + a 2 4 ) \implies c^2 + b^2 = 2 ( d^2 + \frac{a^2}{4})

c 2 + b 2 = 2 × 4 d 2 + a 2 4 \implies c^2+ b^2 = 2 \times \frac{4d^2+a^2}{4}

c 2 + b 2 = 4 d 2 + a 2 2 \implies c^2 + b^2 = \frac{4d^2+a^2}{2}

4 d 2 = 2 ( b 2 + c 2 ) a 2 \implies 4d^2= 2(b^2+c^2)-a^2

4 d 2 = 2 ( a 2 + b 2 + c 2 ) 3 a 2 ( I ) \implies 4d^2 = 2(a^2+b^2+c^2) -3a^2 \ldots \ldots \ldots (I) .

Similarly,

4 e 2 = 2 ( a 2 + b 2 + c 2 ) 3 b 2 ( I I ) 4e^2 = 2(a^2+b^2+c^2) -3b^2 \ldots \ldots \ldots (II) .

4 f 2 = 2 ( a 2 + b 2 + c 2 ) 3 c 2 ( I I I ) 4f^2 = 2(a^2+b^2+c^2) -3c^2 \ldots \ldots \ldots (III) .


As a > b > c a > b > c , we can conclude from ( I ) , ( I I ) (I), (II) and ( I I I ) (III) that

4 d 2 < 4 e 2 < 4 f 2 4d^2 < 4e^2 < 4f^2

d < e < f \implies d < e < f .


Yes, The larger side supports the smaller median in a triangle . So, f \boxed{f} is the answer.

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