Inspired by Brandon Monsen

Geometry Level 5

A right triangle has legs a a and b b , and hypotenuse c c .

The length of the median to the hypotenuse is a 3 b + b 3 a 3 \sqrt[3]{a^{3}b+b^{3}a} .

If H \mathcal{H} is the minimum possible value of c c , find the value of 16 H 16 \mathcal{H} .


Inspiration .


The answer is 4.

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5 solutions

Laurent Shorts
Apr 8, 2016

c 2 = a 3 b + b 3 a 3 c 3 8 = ( a 2 + b 2 ) a b c = 8 a b \frac{c}{2}=\sqrt[3]{a^3b+b^3a} \Leftrightarrow \frac{c^3}{8}=(a^2+b^2)ab \Leftrightarrow c=8ab

Let θ \theta be the angle between a a and c c . Then, c = 8 c 2 cos ( θ ) sin ( θ ) c = 1 8 cos ( θ ) sin ( θ ) c = 1 4 sin ( 2 θ ) \displaystyle c=8c^2\cos(\theta)\sin(\theta) \Leftrightarrow c=\frac{1}{8\cos(\theta)\sin(\theta)} \Leftrightarrow c=\frac{1}{4\sin(2\theta)} . It has a maximum 1 4 \boxed{\frac{1}{4}} when θ = π 4 \theta=\frac{\pi}{4} .

Well, I was going to post my method, but yours is way nicer than mine, so +1 from me!

Brandon Monsen - 5 years, 2 months ago

Again , bashing!!

Let the vertex containing right angle is ( 0 , 0 ) (0,0) . Others be ( a , 0 ) , ( 0 , b ) (a,0),(0,b) .

Now mid point of hypotenuse is ( a / 2 , b / 2 ) (a/2,b/2) .

Using distance formulae and above given info,

c = 8 a b c=8ab .

Or a 2 + b 2 = 64 ( a b ) 2 a^{2} + b^{2}=64(ab)^{2} .

Now by A M G M AM-GM

a 2 + b 2 2 ( a b ) a^{2} + b^{2} \geq 2(ab) .

Therefore a b 1 / 32 ab\geq 1/32 .

Hence c m i n = 1 / 4 = H c_{min} = 1/4= H .

Therefore answer is 4 \boxed{4} .

Andreas Wendler
Apr 8, 2016

Cosine rule for medium m delivers: m 2 = c 2 4 + a 2 2 a c 2 c o s β = ( a 3 b + b 3 a ) 2 3 m^{2}=\frac{c^{2}}{4}+a^{2}-2a\frac{c}{2}cos\beta=(a^{3}b+b^{3}a)^{\frac{2}{3}}

With c o s β = a c cos\beta=\frac{a}{c} we get c 2 = a 2 + b 2 = 4 ( a 3 b + b 3 a ) 2 3 c^{2}=a^{2}+b^{2}=4(a^{3}b+b^{3}a)^{\frac{2}{3}}

That means we have to minimize c 2 = a 2 + b 2 c^{2}=a^{2}+b^{2} regarding constraint a 2 + b 2 = 4 ( a 3 b + b 3 a ) 2 3 a^{2}+b^{2}=4(a^{3}b+b^{3}a)^{\frac{2}{3}}

This task is solved by the well known Lagrange multiplier method which relults in H=0.25.

What is the Lagrange multiplier method?How can it be applied to this?

Upamanyu Mukharji - 5 years, 2 months ago

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It is the determination of extrema for functions with more variables if these variables have to fulfill condition(s) like in the case given!

Andreas Wendler - 5 years, 2 months ago

Is there any solution which does not require trigonometry?

Upamanyu Mukharji - 5 years, 2 months ago

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You can get 8ab =c, then apply AM-GM on a^2 + b^2, and put values in terms of c

Dev Sharma - 5 years, 2 months ago

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Yeah that is much more easy ^^

Khoa Đăng - 5 years ago

How can you solve a problem avoiding the specific of it???

Andreas Wendler - 5 years, 2 months ago

I have an elementary solution.

I'll post it when I am free.

Harsh Shrivastava - 5 years, 2 months ago
Tapas Mazumdar
Jan 26, 2017

Length of median from the vertex C C of Δ A B C \Delta ABC with opposite side length c c and the other two side lengths a a and b b is given by

M = 1 2 2 a 2 + 2 b 2 c 2 M = \dfrac 12 \sqrt{2a^2+2b^2-c^2}

Considering the case of our right triangle, we have c 2 = a 2 + b 2 c^2 = a^2 + b^2 so that

M = 1 2 a 2 + b 2 M = \dfrac 12 \sqrt{a^2+b^2}

Now,

1 2 a 2 + b 2 = a 3 b + b 3 a 3 a 2 + b 2 = 2 a b 3 a 2 + b 2 3 a 2 + b 2 6 = 2 a b 3 c 3 = 2 a b 3 c = 8 a b \begin{aligned} & \dfrac 12 \sqrt{a^2+b^2} &=& \sqrt[3]{a^3b+b^3a} \\ \implies & \sqrt{a^2+b^2} &=& 2\sqrt[3]{ab} \cdot \sqrt[3]{a^2+b^2} \\ \implies & \sqrt[6]{a^2+b^2} &=& 2\sqrt[3]{ab} \\ \implies & \sqrt[3]{c} &=& 2\sqrt[3]{ab} \\ \implies & c &=& 8ab \end{aligned}

Using AM-GM inequality, we have

a 2 + b 2 2 a 2 b 2 c 2 2 a b 64 a 2 b 2 2 a b 32 a b 1 a b 1 32 8 a b 1 4 c 1 4 \begin{aligned} & \dfrac{a^2+b^2}{2} & \ge & \sqrt{a^2b^2} \\ \implies & \dfrac{c^2}{2} & \ge & ab \\ \implies & \dfrac{64a^2b^2}{2} & \ge & ab \\ \implies & 32ab & \ge & 1 \\ \implies & ab & \ge & \dfrac{1}{32} \\ \implies & 8ab & \ge & \dfrac{1}{4} \\ \implies & c & \ge & \dfrac 14 \end{aligned}

So the minimum value of c c is H = 1 4 \mathcal{H} = \dfrac 14 and thus 16 H = 4 16\mathcal{H} = \boxed{4} .

Nice solution !

Harsh Shrivastava - 4 years, 4 months ago

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Thanks friend!

Tapas Mazumdar - 4 years, 4 months ago

T h e m e d i u m = 1 2 c . F o r m i n i m u m c , l e g s m u s t b e e q u a l . S o a = b . a 2 + b 2 = 2 a 2 = c 2 . A n d a = c 2 . 1 2 c = a 3 b + a b 3 3 . ( c 2 ) 3 = 2 a 4 = 2 ( c ( 2 ) 4 ) . c 3 8 = 2 c 4 4 . c = 1 4 = H . S o 16 H = 16 1 4 = 4. The~medium~=~\frac 1 2 c. ~~~~~~For ~minimum~ c,~legs~ must~ be~ equal.~~So~a=b.~~~~ ~~a^2+b^2=2a^2=c^2.~~~~And~~a=\dfrac c {\sqrt2}.\\ \implies~\frac 1 2 c=\sqrt[3]{a^3b+ab^3}.~~~~~~~\Big(\dfrac c 2 \Big)^3=2a^4=2\Big(\dfrac c {(\sqrt2)^4 }\Big).\\ \therefore~\dfrac{c^3} 8=2*\dfrac{c^4}{4}.~~\implies~c=\dfrac 1 4=H.\\ So~~16*H=16*\frac 1 4=\Large ~~~~\color{#D61F06}{4}.

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