Why do this
calls for simple geometrical reasoning (nothing more complicated
than similar triangles) and leads to an obvious generalisation.
Finding the solution is the beginning of the next stage:
how can I explain my method?
is this the best method?
what if...can I generalise this result?
What do you know about the tangent to a circle and its
What do you know about the perpendicular from the centre of a
circle to a chord?
Can you spot any similar triangles?
If two sides of a right-angled triangle are in the ratio 3:5 what
can we say about the third side?
The Eyeball Theorem