What happens in special relativity when the speed v approaches c?

Here are the two formulas I know about special relativity:

t = 1/(√(1 - v² /c² ))(t' + (v /c² )x')

x = 1/(√(1 - v² /c² ))(x' + vt')

I know that when v/c approaches 0, it becomes Newtonian physics. But what happens when v/c approaches 1 in these formulas?