If cos x = p2/1, then tan x =?

If you draw a right angled triangle and put the know things on it you will easily understand what Chris has done.

If you need visual help then ask for it!

If cos x = p2/1, then tan x

Using the Pythagorean Theorem, 

y = √[1 - (p2)^2]

So tan x = √[1 - (p2)^2] / p2

Note.....depending upon the quadrant where x falls, this could be negative, as well