The peak value of the triangular wave is V0.
The equation of the line in the interval 0 to T4 is given as,
V=4V0Tt
The rms value of the triangular wave is given as,
Vrms= ⎷4T∫T40(V)2dt
= ⎷4T×(4V0T)2∫T40(t)2dt
= ⎷(64V02T3)(t33)T40
=V0√3
Thus the rms value of V in the time interval 0 to T4 is V0√3.