5cm
3,4,5 triangle is a well known way of creating a right angle.
Mathmatically,
hypotenuse (a) = square root of b squared plus c squared.
Square root of 9 + 16 = 5
clearly explained.
Also if this is for GCSE maths or equivalent then this is taught as one of the standard ratios you should just know without having to worry and panic
about the working out.
I don't think he was asking for the answer 
That's from some of the "true exam answers given by teenagers" list somewhere on teh
web.
durrr, seems I had a logic and humour bypass on this one.
I'll get my coat.
In a practical sense knowing a 3, 4, 5 triangle can save lots of hassle. Good for checking if walls are square (ie when fitting kitchen worktops) or
for laying out paving or floor tiles.
Strangely the same rule works out if you need a bigger triangle - for instance 0.3m x 0.4m x 0.5m or if you need a big imperial one such as 6' x
8' x 10' etc.
quote:
Originally posted by mistergrumpy
I don't think he was asking for the answer