### Cosines Rule

Three points A, B and C lie in this order on a line, and P is any point in the plane. Use the Cosine Rule to prove the following statement.

### DOTS Division

Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.

### Root to Poly

Find the polynomial p(x) with integer coefficients such that one solution of the equation p(x)=0 is $1+\sqrt 2+\sqrt 3$.

# Divisible Expression

##### Stage: 4 Short Challenge Level:
\eqalign{(1+x+y)^2-(1-x-y)^2&=(1+2(x+y)+(x+y)^2)-(1-2(x+y)+(x+y)^2)\\&=4(x+y)}.

This problem is taken from the UKMT Mathematical Challenges.
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