Line voltage and phase voltage in three phase electricity

This page gives a definition of phase and line voltage in three phase
electricity, and gives the relationship between the rms values of
phase voltage and line voltage.

In three phase electricity, there are three separate voltages
oscillating at the same frequency, but with each one oscillating out
of step with the other two. If the first voltage is $V_0\cos\theta$,
the second voltage oscillates as $V_0\cos(\theta+120^\circ)$, and the
third voltage oscillates as $V_0\cos(\theta+240^\circ)$.

The phase voltage is the voltage difference between neutral and
a single phase. The line voltage is the voltage difference
between two of the phases.

If the phase voltage oscillates as $V_0\cos\theta$ then its root mean square (rms) voltage over one cycle, from $\theta=0$ to $2\pi$, is

The integral is simply ${\pi}$ because the term $\cos2\theta$ evaluates to
zero over this range.

If the phase voltage of line A is $V_0\cos\theta$, and the phase
voltage of line B is $V_0\cos(\theta+2\pi/3)$, then the voltage
between lines A and B is