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Question of Three Sides

Age 14 to 16
ShortChallenge Level Yellow starYellow star
Secondary curriculum
  • Problem
  • Solutions

Using Pythagoras' Theorem on two parts of the triangle
The area of a triangle is $\frac{1}{2}$base$\times$height, so taking the base of the triangle as the side of length $22,$ drawing on the height splits the triangle into two right-angled triangles:

  

$\frac{1}{2}\times22\times h=88$
$\Rightarrow 11\times h=88$
$\Rightarrow h=8$

The base of the red triangle can be found using Pythagoras' theorem, and then we will know the base of the blue triangle (since they add up to $22$), so we can use Pythagoras' Theorem again to find $y.$

$x^2+8^2=10^2$
$\Rightarrow x^2 =10^2-8^2 = 36$
so $x=6$

So the base of the blue triangle must be $16.$

So $8^2+16^2=y^2$
$\Rightarrow 320=y^2$
$\Rightarrow y=\sqrt{320},$
which is equal to $8\sqrt{5}$ or $17.89$ (2 d.p.).


Using trigonometry
In the diagram below, the triangle has been labelled to make substitution into the trigonometric formulae easier.
 


Using the formula for the area of a triangle $\frac{1}{2}ab\sin{C}$, $\frac{1}{2}\times10\times22\times\sin{C}=88\Rightarrow 110\sin{C}=88\Rightarrow\sin{C}=0.8,$ so $C=53.13^\text{o}.$

Then using the cosine rule $c^2=a^2+b^2-2ab\cos{C},$ $$\begin{align}&y^2=10^2+22^2-2\times10\times22\times\cos{53.13^\text{o}}\\
\Rightarrow&y^2=100+484-440\times0.6\\
\Rightarrow&y^2=320\\
\Rightarrow&y=\sqrt{320}=8\sqrt{5}\approx17.89\end{align}$$

You can find more short problems, arranged by curriculum topic, in our short problems collection.

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The NRICH Project aims to enrich the mathematical experiences of all learners. To support this aim, members of the NRICH team work in a wide range of capacities, including providing professional development for teachers wishing to embed rich mathematical tasks into everyday classroom practice.

NRICH is part of the family of activities in the Millennium Mathematics Project.

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