Hola.
Aporto otra forma:
\( \displaystyle\lim_{x \to 1}(1-x)\tan\frac\pi2x=\displaystyle\lim_{x \to 1}(1-x)\dfrac{\sin[\frac\pi2-\frac\pi2(1-x)]}{\cos[\frac\pi2-\frac\pi2(1-x)]}=\displaystyle\lim_{x \to 1}(1-x)\frac{\cos\frac\pi2(1-x)]}{\sin\frac\pi2(1-x)}= \)\( \displaystyle\lim_{x \to 1}(1-x)\dfrac{1}{\frac\pi2(1-x)}=\frac2 \pi \)
Saludos
Carmelo