Puedes usar que \( u= \dfrac{\pi}{3} -t \) entonces \( t \to \dfrac{\pi}{3} \) equivale a \( u \to 0 \)
\( \displaystyle \lim_{t \to \frac{\pi}{3}} \dfrac{\sen(t) - \cos(\dfrac{t}{2})}{\frac{\pi}{3} - t } = \lim_{u \to 0} \dfrac{\sen(\frac{\pi}{3} -u ) - \cos(\frac{\pi}{6} - \frac{u}{2})}{u} \).
\( \sen(\frac{\pi}{6}) = \cos(\frac{\pi}{3}) \)
Se adelantó Julio.