נוספו 823 בתים,
14:10, 28 בינואר 2010 <math>
\lim_{x\rightarrow \frac{\pi}{2}} 2xtg(x)-\frac{\pi}{cos(x)}=
\lim_{x\rightarrow \frac{\pi}{2}} \frac{2xsin(x)}{cos(x)}-\frac{\pi}{cos(x)} =
\lim_{x\rightarrow \frac{\pi}{2}} \frac{2xsin(x)-\pi}{cos(x)} =
</math>
<math>
=\lim_{\Delta x\rightarrow 0} \frac{2(\frac{\pi}{2} + \Delta x)sin(\frac{\pi}{2} + \Delta x)-\pi}{cos(\frac{\pi}{2} + \Delta x)} =
\lim_{\Delta x\rightarrow 0} \frac{2(\frac{\pi}{2} + \Delta x)cos(\Delta x)-\pi}{-sin(\Delta x)} =
</math>
<math>
=\lim_{\Delta x\rightarrow 0} \frac{\pi cos(\Delta x)-\pi}{sin(\Delta x)} + 2cos(\Delta x)\frac{\Delta x}{-sin(\Delta x)}=
</math>
<math>
\lim_{\Delta x\rightarrow 0} \frac{\pi [cos(\Delta x)-1]}{\Delta x}\frac{\Delta x}{sin(\Delta x)} + 2cos(\Delta x)\frac{\Delta x}{-sin(\Delta x)}=
\pi \cdot 0 \cdot 1 + 2 \cdot 1 \cdot (-1) = -2
</math>