You may also like

Roots and Coefficients

If xyz = 1 and x+y+z =1/x + 1/y + 1/z show that at least one of these numbers must be 1. Now for the complexity! When are the other numbers real and when are they complex?

Target Six

Show that x = 1 is a solution of the equation x^(3/2) - 8x^(-3/2) = 7 and find all other solutions.

8 Methods for Three by One

This problem in geometry has been solved in no less than EIGHT ways by a pair of students. How would you solve it? How many of their solutions can you follow? How are they the same or different? Which do you like best?

Complex Rotations

Age 16 to 18
Challenge Level
Well done Christopher Tynan, St. Bees School, for this very nice solution and also Andrei Lazanu, School No. 205, Bucharest, Romania and Jesse Rasowsky of Bethlehem Central High School and Matthew Hartley.

Below the complex numbers $3-i$ and $2-10i$ are plotted, as well as their corresponding image when multiplied by -1. the complex numbers are plotted

Our conjecture is that multiplying a point on the Argand diagram by -1 is equivalent to a 180 degree rotation about the origin.

Let $(a,b)$ on the Argand diagram represent the complex number $a+ib$, where $i$ is the square root of -1. So, multiplying $a+ib$ by -1, we get $-a-ib$, which is represented by the point $(-a,-b)$ in the Argand diagram. However, the point $(-a,-b)$ is a rotation of the point $(a,b)$ by 180 degrees about the origin. QED.

Using the same numbers for the second part, our conjecture here is that multiplying a complex number by $i$ gives a rotation of 90 degrees anti-clockwise on the Argand diagram.
Graph Plot
Again, assuming $(a,b)$ is a complex number plotted on the Argand diagram, by multiplying $a+ib$ by $i$, we get the complex number $-b+ai$, which is represented by the point $(-b,a)$ on the Argand diagram. This is equivalent to a rotation of 90 degrees anti-clockwise about the origin. QED.

Assuming we were to multiply the number by $i$ twice, this would give a rotation of 180 degrees (as shown in the first part because $i^2=-1$). Multiplying by $i$ thrice would be equivalent to a rotation of 270 degrees anti-clockwise, or 90 degrees clockwise and four time maps the number onto itself (essentially multiplying by 1). Multiplying by $i$ $n$ times is equivalent to rotating the point 90 degrees anti-clockwise $n$ times. Therefore; if $n$ is 0 mod 4, it has no effect; if $n$ is 1 mod 4, it's equivalent to an anti-clockwise rotation of 90 degrees; if $n$ is 2 mod 4, it's equivalent to a rotation of 180 degrees; if $n$ is 3 mod 4, it's equivalent to a rotation of 90 degrees clockwise.

NB. All these rotations are rotations of the initial point (a,b) about the origin as the centre of rotation.