A 'doodle' is a closed intersecting curve drawn without taking
pencil from paper. Only two lines cross at each intersection or
vertex (never 3), that is the vertex points must be 'double points'
not 'triple points'. Number the vertex points in any order.
Starting at any point on the doodle, trace it until you get back to
where you started. Write down the numbers of the vertices as you
pass through them. So you have a [not necessarily unique] list of
numbers for each doodle. Prove that 1)each vertex number in a list
occurs twice. [easy!] 2)between each pair of vertex numbers in a
list there are an even number of other numbers [hard!]
How many different cubes can be painted with three blue faces and
three red faces? A boy (using blue) and a girl (using red) paint
the faces of a cube in turn so that the six faces are painted in
order 'blue then red then blue then red then blue then red'. Having
finished one cube, they begin to paint the next one. Prove that the
girl can choose the faces she paints so as to make the second cube
the same as the first.
Factorial one hundred (written 100!) has 24 noughts when written in full and that 1000! has 249 noughts? Convince yourself that the above is true. Perhaps your methodology will help you find the number of noughts in
10 000! and 100 000! or even 1 000 000!
A walk is made up of diagonal steps, starting at the bottom left
and ending up back on the bottom line (x-axis). You can move
diagonally up and down towards the right but you cannot move towards the left.
A diagonal must go from a left-hand corner of a square to the
opposite right-hand corner of the same square.
The examples above all show 10-step walks. So let's look at a
2-step walk in more detail:
There is only one
way to make a 2-step walk - from A up to x and down to B:
Here are two 4-step walks:
Are there any more 4-step walks?
How do you know you have them all?
How many 6-step, 8-step walks are there?
Can you find a general rule?