I hope someone can help me with this problem:

If y= log {x + (1 + x^2)^1/2} what is dy/dx?

apprantly the answer is dy/dx = (1 + x^2)^-1/2 , but I cannot see how to do it. Is it a function of a function? My lecturer showed us this on the board - but I still can't understand - so I would appreciate it if someone could help - thank you.