### Real(ly) Numbers

If x, y and z are real numbers such that: x + y + z = 5 and xy + yz + zx = 3. What is the largest value that any of the numbers can have?

### Overturning Fracsum

Can you solve the system of equations to find the values of x, y and z?

### Building Tetrahedra

Can you make a tetrahedron whose faces all have the same perimeter?

# Always Two

##### Age 14 to 18Challenge Level

When $b=1$, you can substitute this into the original equations to get a set of two equations in $a$ and $c$.  With a bit of manipulation you can turn these into a quadratic equation in either $a$ or $c$.  Solve this to find some possible values.

When $a=c$ you can rewrite $c$ as $a$ in the original equations, giving you two equations in $a$ and $b$. You can eliminate $b$ to get a cubic equation in $a$. Use the work you have already done in the previous case to find a possible solution for $a$ and use this to factorise the cubic into a product of a linear and quadratic term.

In the video below Claire shows you how you could factorise a cubic.