Ontario 10 Academic (MPM2D)
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Triangle medians (Investigation)
Lesson

How can we find the length of the median, $m$m, given the length of the three sides of a $\triangle ABC$ABC $a,b$a,b and $c$c?

HINT for STEP 1 - Can you turn $\triangle ABC$ABC into a parallelogram  $ABA'C$ABAC?  Try this first, then read on to see how you went.

By using $\triangle ABC$ABC and recognising that this forms exactly half of the parallelogram, we copy the triangle and rotate it 180 degrees.

This results in the following parallelogram.

From here can you identify all the equalities and similarities that exist?  There are a lot! Being able to identify these really helps in trying to construct proofs or logical arguments.  Particularly can you see the lengths of the $2$2 diagonals?

We can see that the diagonal $BC$BC has length $a$a, as we were told the lengths in the original statement. Also we can see the diagonal $AA'$AA has length $2m$2m

There is a property of parallelograms that states that the sum of squares of the sides of a parallelogram is equal to the sum of the squares of the diagonals, (if you wish to see this proof go here).   

So we can say then that $(2m)^2+a^2=2b^2+2c^2$(2m)2+a2=2b2+2c2

This means that the length of the median, m is 

$m_a=\frac{\sqrt{2b^2+2c^2-a^2}}{2}$ma=2b2+2c2a22

 

We can follow the same process and find the medians $m_b$mb, and $m_c$mc.  Can you find these expressions?  Try this before continuing on to check your answer.

$m_b=\frac{\sqrt{2c^2+2a^2-b^2}}{2}$mb=2c2+2a2b22

$m_c=\frac{\sqrt{2a^2+2b^2-c^2}}{2}$mc=2a2+2b2c22

 

What is the sum of the squares of the medians?  

$m_a^2+m_b^2+m_c^2$m2a+m2b+m2c $=$= $(\frac{\sqrt{2b^2+2c^2-a^2}}{2})^2+(\frac{\sqrt{2c^2+2a^2-b^2}}{2})^2+(\frac{\sqrt{2a^2+2b^2-c^2}}{2})^2$(2b2+2c2a22)2+(2c2+2a2b22)2+(2a2+2b2c22)2
  $=$= $(\frac{2b^2+2c^2-a^2}{4})+(\frac{2c^2+2a^2-b^2}{4})+(\frac{2a^2+2b^2-c^2}{4})$(2b2+2c2a24)+(2c2+2a2b24)+(2a2+2b2c24)
  $=$= $\frac{3a^2+3b^2+3c^2}{4}$3a2+3b2+3c24
  $=$= $\frac{3}{4}\left(a^2+b^2+c^2\right)$34(a2+b2+c2)

Whilst this doesn't really have any real life applications, it is a rather neat result.  That for any triangle, the sum of the squares of the medians is 3 quarters of the sum of the squares of the sides. 

Outcomes

10D.AG3.01

Determine, through investigation, some characteristics and properties of geometric figures

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