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## Monday, 14 April 2014

### Sunday Afternoon Maths VIII Answers & Extensions

This post contains the answers to this week's Sunday Afternoon Maths and some extension problems based around the originals.

#### Two

a+(a+A)-1 = a + A = 2.
So, a+2-1 = 2
So, a = 3/2
By the same method, b = 2-√2, c = -2 and d = 2-2k
##### Extension
For which values of k does this converge?

#### Reverse Bases Again

445 in base 10 is equal to 544 in base 7
##### Extension
Find another pair of bases A and B so that there exist digits d, e and f such that def in base A is equal to fed in base B?

A=8, B=9 and C=7.
##### Extension
Find bases A, B and C such that 123A+456B=789C

#### Complex Squares

Any complex number can be written in the form z=reiθ.
This gives that z2=r2e2iθ, which will have positive real and complex parts when 0+2πn < 2θ < π/2+2πn.
This will occur when 0 < θ < π/4 and π < θ < /4.
This can be represented on an Argand diagram:
A complex number z falls in these regions when |Re(z)|>|Im(z)| and sign(Re(z))=sign(Im(z))
##### Extension
For which complex numbers, z, are Re(z3) and Re(z3) both positive?

#### Rebounds

If mirrors were placed along the walls of the rectangle, the ball would appear to travel in a straight line across a grid of rectangles:
Viewed this way, the ball will still stop once it reaches a corner:
For an n by m (in above example: n = 4, m = 3) rectangle, this will occur once the ball has travelled through lcm(n, m) squares.
 Let a = lcm(n, m) n
 Let b = lcm(n, m) m
On its way to the corner, the ball will bounce b – 1 times off the top and bottom and a – 1 times off the sides. It can be seen that if a – 1 is even, then the ball will end in one of the corners on the right hand side. The complete results can be seen in the following Carroll diagram:
 a – 1 odd a – 1 even b – 1 odd Top left Top right b – 1 even Bottom left Bottom right
It can be shown that the ball will never finish in the top left (where it started) as this would require it to travel through the bottom right first. Therefore the following holds:
 a even a odd b even Top right b odd Bottom left Bottom right
##### Extension
For which sizes of rectangle will the path of the ball make the same pattern?