A sequence of numbers x1, x2, x3, ... starts with x1 = 2, and, if
you know any term xn, you can find the next term xn+1 using the
formula: xn+1 = (xn + 3/xn)/2 . Calculate the first six terms of
this sequence. What do you notice? Calculate a few more terms and
find the squares of the terms. Can you prove that the special
property you notice about this sequence will apply to all the later
terms of the sequence? Write down a formula to give an
approximation to the cube root of a number and test it for the cube
root of 3 and the cube root of 8. How many terms of the sequence do
you have to take before you get the cube root of 8 correct to as
many decimal places as your calculator will give? What happens when
you try this method for fourth roots or fifth roots etc.?

When in 1821 Charles Babbage invented the `Difference Engine' it
was intended to take over the work of making mathematical tables by
the techniques described in this article.

Can you think like a computer and work out what this flow diagram
does?

A Fine Thing?

Age 16 to 18 Challenge Level:

This challenge cipher forms part of a very difficult sequence of ciphers suitable for keen groups or individuals, maths clubs and very optional homework challenges. Don't try this in the classroom!