Find the maximum value in a triangle

Geometry Level 5

The acute A B C \triangle ABC is such that:

  • h A h_A , h B h_B , and h C h_C are the lengths of the altitudes from A A , B B , and C C respectively.
  • m A m_A , m B m_B , and m C m_C are the lengths of the medians from A A , B B , and C C respectively.
  • The lengths of the radii of the circumcircle and incircle of the A B C \triangle ABC are 6 6 and 2 2 respectively.

Find the smallest integer upper bound of m A h A + m B h B + m C h C . \dfrac{m_A}{h_A}+\dfrac{m_B}{h_B}+\dfrac{m_C}{h_C} .


The answer is 4.

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1 solution

Sathvik Acharya
Nov 15, 2020

Let r r , R R and S S represent the inradius , circumradius and area of the triangle respectively.

Claim: 1 h A + 1 h B + 1 h C = 1 r \frac{1}{h_A}+\frac{1}{h_B}+\frac{1}{h_C}=\frac{1}{r} Proof: 1 h A + 1 h B + 1 h C = a 2 S + b 2 S + c 2 S = a + b + c 2 S = 1 r \frac{1}{h_A}+\frac{1}{h_B}+\frac{1}{h_C}=\frac{a}{2S}+\frac{b}{2S}+\frac{c}{2S}=\frac{a+b+c}{2S}=\frac{1}{r}

Let O O the center of the circumcircle of A B C \triangle ABC and D , E , F D, E, F be the midpoints of side B C , C A , A B BC, CA, AB respectively.

Claim: O D h A + O E h B + O F h C = 1 \frac{OD}{h_A}+\frac{OE}{h_B}+\frac{OF}{h_C}=1 Proof: O D a + O E b + O F c = a h A = b h B = c h C = 2 S OD\cdot a + OE\cdot b + OF\cdot c = a\cdot h_A = b\cdot h_B = c\cdot h_C=2S O D h A + O E h A + O F h C = 1 \implies \frac{OD}{h_A}+\frac{OE}{h_A}+\frac{OF}{h_C}=1

Also, m A = A D O A + O D m A R + O D m_A =AD \le OA+OD \implies m_A\le R+OD . Writing similar inequalities and adding up we get, m A h A + m B h B + m C h C R ( 1 h A + 1 h B + 1 h C ) + ( O D h A + O E h A + O F h C ) \frac{m_A}{h_A}+\frac{m_B}{h_B}+\frac{m_C}{h_C} \le R\left (\frac{1}{h_A}+\frac{1}{h_B}+\frac{1}{h_C} \right ) + \left(\frac{OD}{h_A}+\frac{OE}{h_A}+\frac{OF}{h_C} \right )

Using the above claims we get the well known inequality, m A h A + m B h B + m C h C 1 + R r \boxed{\frac{m_A}{h_A}+\frac{m_B}{h_B}+\frac{m_C}{h_C} \le 1+\frac{R}{r}}

Therefore the maximum value of m A h A + m B h B + m C h C \frac{m_A}{h_A}+\frac{m_B}{h_B}+\frac{m_C}{h_C} is 4 \boxed{4}

@Sathvik Acharya I think you meant O D h A + O E h B + O F h c \dfrac {OD}{h_A} + \dfrac {OE}{\color{#D61F06} h_B}+\dfrac {OF}{h_c} in the second claim.

N. Aadhaar Murty - 6 months ago

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Yes, I have made the necessary corrections. Thank you!

Sathvik Acharya - 6 months ago

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Your welcome. Just one more question - you have assumed that we are in an acute angled triangle, but the question doesn't state that anywhere. @Sathvik Acharya

N. Aadhaar Murty - 5 months, 4 weeks ago

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@N. Aadhaar Murty You are right, the proof holds only for acute triangles, also I am not sure if equality can be achieved.

Sathvik Acharya - 4 months, 2 weeks ago

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