# Trying to understand velocity addition and time dilation [duplicate]

I'm familiar with Einstein's formulae $$V=\frac{u+v}{1+ \frac{uv}{c^2}}$$ and $$\Delta t'=\frac{\Delta t}{\sqrt{1-\frac{v^2}{c^2}}}$$, the former being velocity addition and the latter being time dilation. Each of these equations imply that you can't go faster than light; the former cannot exceed $$c$$, and the latter will give an imaginary number in the denominator if you try.

But why are these formulae true? I understand it numerically, but how would you explain this conceptually?