Leading-order error terms in mean values (1)

Weighted arithmetic mean: $$ \begin{align} [\overline{u}^{t,\theta}]^{n+\theta} & = \theta u^{n+1} + (1-\theta)u^n = u(t_{n+\theta}) + R^{n+\theta}, \tag{17}\\ R^{n+\theta} &= {\half}u''(t_{n+\theta})\Delta t^2\theta (1-\theta) + \Oof{\Delta t^3} \tp \tag{18} \end{align} $$ Standard arithmetic mean: $$ \begin{align} [\overline{u}^{t}]^{n} &= \half(u^{n-\half} + u^{n+\half}) = u(t_n) + R^{n}, \tag{19}\\ R^{n} &= \frac{1}{8}u''(t_{n})\Delta t^2 + \frac{1}{384}u''''(t_n)\Delta t^4 + \Oof{\Delta t^6}\tp \tag{20} \end{align} $$