![image](https://tex.z-dn.net/?f=%5Csqrt%7B2%7Dtgx%5Ccdot+ctgx%3C2%5Csin+x%3B%5C%5C%0AD%28f%29%3A+%5Cleft+%5C%7B+%7B%7Bx%5Cneq%5Cpi+n%7D+%5Catop+%7Bx%5Cneq%5Cfrac%5Cpi2%2B%5Cpi+k%7D%7D+%5Cright.+n%2Ck%5Cin+Z%3B%5C+%5C+%3D%3D%3Ex%5Cneq%5Cfrac%7B%5Cpi+n%7D%7B2%7D%2C%5C+n%5Cin+Z%3B%5C%5C%0AD%28f%29%3Ax%5Cin%5Cleft%28%5Cfrac%7B%5Cpi+n%7D%7B2%7D%3B%5Cfrac%5Cpi2%2B%5Cfrac%7B%5Cpi+n%7D%7B2%7D%5Cright%29%2C%5C+n%5Cin+Z%3B%5C%5C%0A%5Csqrt2%3C2%5Csin+x%3B%5C%5C%0A%5Cfrac%7B%5Csqrt2%7D%7B2%7D%3C%5Csin+x%3B%5C%5C%0A%5Csin+x%3D%5Cfrac%7B%5Csqrt2%7D%7B2%7D%3B%5C%5C%0Ax%3D%5Cleft%28-1%5Cright%29%5En%5Ccdot%5Cfrac%5Cpi4%2B%5Cpi+n%3D+%5Cleft+%5C%7B+%7B%7Bx%3D%5Cfrac%5Cpi4%2B2%5Cpi+l%7D+%5Catop+%7Bx%3D%0A-%5Cfrac%7B%5Cpi%7D%7B4%7D%2B%5Cpi%2B2%5Cpi+m%7D%7D+%5Cright.%3D%3D%3E%5C%5C%0A%3D%3D%3E++%5Cleft+%5C%7B+%7B%7Bx%3D%5Cfrac%5Cpi4%2B2%5Cpi+n%7D+%5Catop+%7Bx%3D%0A%5Cfrac%7B3%5Cpi%7D%7B4%7D%2B2%5Cpi+m%7D%7D+%5Cright.+%5C+n%2Cm%5Cin+Z%3B%5C%5C)
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">