### Right Time

At the time of writing the hour and minute hands of my clock are at right angles. How long will it be before they are at right angles again?

### Isosceles

Prove that a triangle with sides of length 5, 5 and 6 has the same area as a triangle with sides of length 5, 5 and 8. Find other pairs of non-congruent isosceles triangles which have equal areas.

Four rods, two of length a and two of length b, are linked to form a kite. The linkage is moveable so that the angles change. What is the maximum area of the kite?

##### Age 11 to 14 Short Challenge Level:
Using interior angles of triangles
In the diagram below, all of the lines have been extended so that they cross.

b, c and the red angle form a triangle so add up to 180$^\text o$.
The same is true of f, g and the pink angle and of j, k and the orange angle.

So adding up all of the angles in all 3 triangles gives
b + c + red angle + f + g + pink angle + j + k + orange angle = 180$^\text o$ + 180$^\text o$ + 180$^\text o$

But the red angle, the pink angle and the orange angle are also the angles of a large triangle. So they also add up to 180$^\text o$.

That leaves b + c + f + g + j + k = 180$^\text o$ + 180$^\text o$ = 360$^\text o$.
Using the diagram below in the same way, l + a + d + e + h + i is also equal to 360$^\text o$.
So the sum of all of the angles denoted by letters is 360$^\text o$ + 360$^\text o$ = 720$^\text o$.

Using the exterior angles of hexagons
Imagine walking clockwise around the hexagon shown in red in the diagram on the right. The angles a, k, i, g, e and c are the angles that you would need to turn through to be facing in the direction of the next side.

By the time you got back to where you started from, you would be facing the same way you were facing when you started. So you would have turned through 360$^\text o$.

So a + k + i + g + e + c = 360$^\text o$.

Now imagine walking anticlockwise around the hexagon, as shown in green.

Similarly,
b + d + f + h + j + l = 360$^\text o$.

So the sum of all of the angles denoted by letters is
360$^\text o$ + 360$^\text o$ = 720$^\text o$.
You can find more short problems, arranged by curriculum topic, in our short problems collection.