x\neq\frac{\pi n}{2},\ n\in Z;\\
D(f):x\in\left(\frac{\pi n}{2};\frac\pi2+\frac{\pi n}{2}\right),\ n\in Z;\\
\sqrt2<2\sin x;\\
\frac{\sqrt2}{2}<\sin x;\\
\sin x=\frac{\sqrt2}{2};\\
x=\left(-1\right)^n\cdot\frac\pi4+\pi n= \left \{ {{x=\frac\pi4+2\pi l} \atop {x=
-\frac{\pi}{4}+\pi+2\pi m}} \right.==>\\
==> \left \{ {{x=\frac\pi4+2\pi n} \atop {x=
\frac{3\pi}{4}+2\pi m}} \right. \ n,m\in Z;\\" alt="\sqrt{2}tgx\cdot ctgx<2\sin x;\\
D(f): \left \{ {{x\neq\pi n} \atop {x\neq\frac\pi2+\pi k}} \right. n,k\in Z;\ \ ==>x\neq\frac{\pi n}{2},\ n\in Z;\\
D(f):x\in\left(\frac{\pi n}{2};\frac\pi2+\frac{\pi n}{2}\right),\ n\in Z;\\
\sqrt2<2\sin x;\\
\frac{\sqrt2}{2}<\sin x;\\
\sin x=\frac{\sqrt2}{2};\\
x=\left(-1\right)^n\cdot\frac\pi4+\pi n= \left \{ {{x=\frac\pi4+2\pi l} \atop {x=
-\frac{\pi}{4}+\pi+2\pi m}} \right.==>\\
==> \left \{ {{x=\frac\pi4+2\pi n} \atop {x=
\frac{3\pi}{4}+2\pi m}} \right. \ n,m\in Z;\\" align="absmiddle" class="latex-formula">