### Flexi Quads

A quadrilateral changes shape with the edge lengths constant. Show the scalar product of the diagonals is constant. If the diagonals are perpendicular in one position are they always perpendicular?

### Flexi Quad Tan

As a quadrilateral Q is deformed (keeping the edge lengths constnt) the diagonals and the angle X between them change. Prove that the area of Q is proportional to tanX.

### Air Routes

Find the distance of the shortest air route at an altitude of 6000 metres between London and Cape Town given the latitudes and longitudes. A simple application of scalar products of vectors.

# Cross with the Scalar Product

##### Stage: 5 Challenge Level:

Consider the vector
$${\bf v}=\pmatrix{1\cr 2\cr 3}$$
Investigate the properties of vectors ${\bf u}$ such that ${\bf u}\cdot {\bf v}=0$.
Describe geometrically the set of all such vectors ${\bf u}$.

Now explore the possibilities for vectors ${\bf w}$ which are the result of taking the vector cross product of ${\bf v}$ with another vector. How does this relate to the first part of the question?

Which of these vectors could arise from taking the vector cross product of ${\bf v}$ with another vector? Before performing lots of algebra, can you work out a quick way to make your decision?
$${\bf w}=\pmatrix{0\cr 3\cr -2}\,, \pmatrix{795\cr 11\cr 167}, \pmatrix{1\cr -1\cr 0} \mbox{ or } \pmatrix{-7\cr -7\cr 7}$$

Can you find a way of quickly constructing other such vectors?