(x->pi)
x-π=t
x=π+t
[m]=\lim_{t \to 0 }(-t)\cdot tg\frac{\pi+t}{2}=-\lim_{t \to 0 }t\cdot tg(\frac{\pi}{2}+\frac{t}{2})=-\lim_{t \to 0 }t\cdot (-ctg\frac{t}{2})[/m]
[m]=\lim_{t \to 0 }\frac{t}{tg\frac{t}{2}}=\lim_{t \to 0 }\frac{2\frac{t}{2}}{tg\frac{t}{2}}=2\lim_{t \to 0 }\frac{\frac{t}{2}}{tg\frac{t}{2}}=2\cdot 1=2[/m]