### Building Tetrahedra

Can you make a tetrahedron whose faces all have the same perimeter?

A 1 metre cube has one face on the ground and one face against a wall. A 4 metre ladder leans against the wall and just touches the cube. How high is the top of the ladder above the ground?

### Areas and Ratios

What is the area of the quadrilateral APOQ? Working on the building blocks will give you some insights that may help you to work it out.

# Quarters

##### Stage: 4 Short Challenge Level:

The diagram shows the top-right-hand portion of the square.
The shaded trapezium is labelled QXYZ and W is the point at which ZY produced meets PQ.
As QXYZ is an isosceles trapezium, ∠QZY = ∠ZQX = 45°.
Also, as YX is parallel to ZQ, ∠XYW = ∠WXY = 45°. So WYX and WZQ are both isosceles right-angled triangles. As ∠ZWQ = 90° and Z is at centre of square PQRS, we deduce that W is the midpoint of PQ. Hence WX = XQ = $\frac{1}{4}$PQ. So the ratio of the side-lengths of similar triangles WYX and WZQ is 1:2 and hence the ratio of their areas 1:4.
Therefore the area of trapezium QXYZ = $\frac{3}{4}$ x area of triangle ZWQ = $\frac{3}{32}$ x area PQRS since triangle ZWQ is one-eighth of PQRS. So the fraction of the square which is shaded is 4 x $\frac{3}{32}$ = $\frac{3}{8}$.

This problem is taken from the UKMT Mathematical Challenges.