As the angular acceleration is constant, we have
θω0t+12αt2=12αt2
Thus, 5 rad = 12α(1s)62
α=10rad/s2 or α=10rad/s2
The angle rotated during the first two seconds is
=12×(10rad/s2)(2s)2=20rad.
Thus, the angle rotated during the 2nd second is
20rad−5rad=15rad.