how we obtain this equation from Newton's second law

Just recall Newton's law of motion:Recall    S =  (v + u)*t / 2. .................(1)Similarly    S will be replaced with θ,                     v  will be replaced with ω                      u will be replaced with ω0Therefore θ  =  (ω + ω0)*t / 2. ..............  (2)Recall from linear motion  v = u + at.For angular motion it would be  ω = ω0 + αt ..............(3)The linear acceleration a, is replaced with angular acceleration αSubstituting (3) into (2).θ = ( ω + ω0) * t/2θ = (ω0 + αt + ω0) *t / 2θ = (2ω0 + αt) * t/2θ = (2ω0t + αt^2) /2θ = (ω0t  +  1/2 * αt^2).      (Q.E.D)