You may also like

Double Time

Crack this code which depends on taking pairs of letters and using two simultaneous relations and modulus arithmetic to encode the message.

Modular Fractions

We only need 7 numbers for modulus (or clock) arithmetic mod 7 including working with fractions. Explore how to divide numbers and write fractions in modulus arithemtic.


What is the smallest perfect square that ends with the four digits 9009?

Rational Round

Age 16 to 18
Challenge Level

Show that for every integer $k$ the point $(x, y)$, where

$$x = {2k\over k^2 + 1}, \ y = {k^2 - 1\over k^2 + 1},$$

lies on the unit circle, $x^2 + y^2 =1$. That is, there are infinitely many rational points on this circle.

Show that there are no rational points on the circle $x^2 + y^2 =3$.