Derive Eq. 5-21 from Eqs. 5-18 and 5-19. In Eq. 5-21 2f = 2i = 2

Derive Eq. 5-21 from Eqs. 5-18 and 5-19.
In Eq. 5-21
ω2f = ω2i = 2α ∆θ
In Eq. 5-18
∆ω = ωf - ωi = α∆t
In Eq. 5-19
∆θ = 1/2(ωf + ωi) ∆t