Given the inequality:
$$x \cos{\left(2 x - \frac{\pi}{4} \right)} \leq - \frac{\sqrt{3}}{2}$$
To solve this inequality, we must first solve the corresponding equation:
$$x \cos{\left(2 x - \frac{\pi}{4} \right)} = - \frac{\sqrt{3}}{2}$$
Solve:
$$x_{1} = 35.7477306320813$$
$$x_{2} = 90.7087141383207$$
$$x_{3} = 92.2889761903236$$
$$x_{4} = 2.57756789869373$$
$$x_{5} = 4.41833218870582$$
$$x_{6} = -41.2228986266421$$
$$x_{7} = -94.6359030607806$$
$$x_{8} = 65.574143045524$$
$$x_{9} = 98.5718624260433$$
$$x_{10} = -88.3523923305876$$
$$x_{11} = -91.4941532834635$$
$$x_{12} = 87.5669502052005$$
$$x_{13} = -69.501507035948$$
$$x_{14} = -17.6959380645662$$
$$x_{15} = -53.7917240434184$$
$$x_{16} = 46.7219223213739$$
$$x_{17} = -80.5086903111165$$
$$x_{18} = -55.3783900020515$$
$$x_{19} = -25.5084617903844$$
$$x_{20} = -14.5596242389401$$
$$x_{21} = -96.2157755104838$$
$$x_{22} = -61.6607785557254$$
$$x_{23} = -99.3572258649787$$
$$x_{24} = -2.16887082755251$$
$$x_{25} = -97.7776427399657$$
$$x_{26} = -77.367316054284$$
$$x_{27} = -66.3596193779403$$
$$x_{28} = 1.48855960517723$$
$$x_{29} = -23.9727106489827$$
$$x_{30} = 64.0167145801539$$
$$x_{31} = -60.0757514718742$$
$$x_{32} = -16.0737101108477$$
$$x_{33} = 5.81575314037901$$
$$x_{34} = -31.7950050422583$$
$$x_{35} = 10.6436031352424$$
$$x_{36} = 68.7160376509593$$
$$x_{37} = 54.5931043261049$$
$$x_{38} = 62.4322180425498$$
$$x_{39} = 12.137966959545$$
$$x_{40} = 18.4333574770289$$
$$x_{41} = -45.9552156096449$$
$$x_{42} = 48.3109505632436$$
$$x_{43} = -82.0688317633173$$
$$x_{44} = -83.6500809710862$$
$$x_{45} = 21.578377129864$$
$$x_{46} = 78.1415757577305$$
$$x_{47} = 43.5796612954408$$
$$x_{48} = -19.2197177647678$$
$$x_{49} = -67.9433144530403$$
$$x_{50} = -11.4262062008546$$
$$x_{51} = 86.0061336494404$$
$$x_{52} = 34.1521398153172$$
$$x_{53} = -39.6735225187304$$
$$x_{54} = 93.850466602772$$
$$x_{55} = 40.4372963456374$$
$$x_{56} = -104.061095462942$$
$$x_{57} = 76.5819752896018$$
$$x_{58} = -8.29895266969519$$
$$x_{59} = -58.5195628953145$$
$$x_{60} = -38.0804389435651$$
$$x_{61} = 49.8640990821956$$
$$x_{62} = 13.7759211770063$$
$$x_{63} = 32.6073049652536$$
$$x_{64} = 79.7233451333755$$
$$x_{65} = -75.7852089599415$$
$$x_{66} = 100.133941444321$$
$$x_{67} = 27.8660932450522$$
$$x_{68} = 56.1482564243358$$
$$x_{69} = -33.3923908048991$$
$$x_{70} = 84.4251735228072$$
$$x_{71} = 7.51899978446191$$
$$x_{72} = -6.61018866689758$$
$$x_{73} = 24.7225236560155$$
$$x_{74} = -47.5074737578972$$
$$x_{75} = 57.7342653899289$$
$$x_{76} = -3.40573797678636$$
$$x_{77} = 70.2992953395131$$
$$x_{78} = -30.2521447011736$$
$$x_{79} = 71.8579058472682$$
$$x_{80} = 26.3272887370679$$
$$x_{81} = -89.9329046251528$$
$$x_{82} = 20.0492573318314$$
$$x_{83} = 42.029105157638$$
$$x_{84} = -22.3644811896294$$
$$x_{85} = -74.2259602830878$$
$$x_{86} = -9.77311232132626$$
$$x_{87} = -52.2372675901086$$
$$x_{88} = -44.3652354315924$$
$$x_{1} = 35.7477306320813$$
$$x_{2} = 90.7087141383207$$
$$x_{3} = 92.2889761903236$$
$$x_{4} = 2.57756789869373$$
$$x_{5} = 4.41833218870582$$
$$x_{6} = -41.2228986266421$$
$$x_{7} = -94.6359030607806$$
$$x_{8} = 65.574143045524$$
$$x_{9} = 98.5718624260433$$
$$x_{10} = -88.3523923305876$$
$$x_{11} = -91.4941532834635$$
$$x_{12} = 87.5669502052005$$
$$x_{13} = -69.501507035948$$
$$x_{14} = -17.6959380645662$$
$$x_{15} = -53.7917240434184$$
$$x_{16} = 46.7219223213739$$
$$x_{17} = -80.5086903111165$$
$$x_{18} = -55.3783900020515$$
$$x_{19} = -25.5084617903844$$
$$x_{20} = -14.5596242389401$$
$$x_{21} = -96.2157755104838$$
$$x_{22} = -61.6607785557254$$
$$x_{23} = -99.3572258649787$$
$$x_{24} = -2.16887082755251$$
$$x_{25} = -97.7776427399657$$
$$x_{26} = -77.367316054284$$
$$x_{27} = -66.3596193779403$$
$$x_{28} = 1.48855960517723$$
$$x_{29} = -23.9727106489827$$
$$x_{30} = 64.0167145801539$$
$$x_{31} = -60.0757514718742$$
$$x_{32} = -16.0737101108477$$
$$x_{33} = 5.81575314037901$$
$$x_{34} = -31.7950050422583$$
$$x_{35} = 10.6436031352424$$
$$x_{36} = 68.7160376509593$$
$$x_{37} = 54.5931043261049$$
$$x_{38} = 62.4322180425498$$
$$x_{39} = 12.137966959545$$
$$x_{40} = 18.4333574770289$$
$$x_{41} = -45.9552156096449$$
$$x_{42} = 48.3109505632436$$
$$x_{43} = -82.0688317633173$$
$$x_{44} = -83.6500809710862$$
$$x_{45} = 21.578377129864$$
$$x_{46} = 78.1415757577305$$
$$x_{47} = 43.5796612954408$$
$$x_{48} = -19.2197177647678$$
$$x_{49} = -67.9433144530403$$
$$x_{50} = -11.4262062008546$$
$$x_{51} = 86.0061336494404$$
$$x_{52} = 34.1521398153172$$
$$x_{53} = -39.6735225187304$$
$$x_{54} = 93.850466602772$$
$$x_{55} = 40.4372963456374$$
$$x_{56} = -104.061095462942$$
$$x_{57} = 76.5819752896018$$
$$x_{58} = -8.29895266969519$$
$$x_{59} = -58.5195628953145$$
$$x_{60} = -38.0804389435651$$
$$x_{61} = 49.8640990821956$$
$$x_{62} = 13.7759211770063$$
$$x_{63} = 32.6073049652536$$
$$x_{64} = 79.7233451333755$$
$$x_{65} = -75.7852089599415$$
$$x_{66} = 100.133941444321$$
$$x_{67} = 27.8660932450522$$
$$x_{68} = 56.1482564243358$$
$$x_{69} = -33.3923908048991$$
$$x_{70} = 84.4251735228072$$
$$x_{71} = 7.51899978446191$$
$$x_{72} = -6.61018866689758$$
$$x_{73} = 24.7225236560155$$
$$x_{74} = -47.5074737578972$$
$$x_{75} = 57.7342653899289$$
$$x_{76} = -3.40573797678636$$
$$x_{77} = 70.2992953395131$$
$$x_{78} = -30.2521447011736$$
$$x_{79} = 71.8579058472682$$
$$x_{80} = 26.3272887370679$$
$$x_{81} = -89.9329046251528$$
$$x_{82} = 20.0492573318314$$
$$x_{83} = 42.029105157638$$
$$x_{84} = -22.3644811896294$$
$$x_{85} = -74.2259602830878$$
$$x_{86} = -9.77311232132626$$
$$x_{87} = -52.2372675901086$$
$$x_{88} = -44.3652354315924$$
This roots
$$x_{56} = -104.061095462942$$
$$x_{23} = -99.3572258649787$$
$$x_{25} = -97.7776427399657$$
$$x_{21} = -96.2157755104838$$
$$x_{7} = -94.6359030607806$$
$$x_{11} = -91.4941532834635$$
$$x_{81} = -89.9329046251528$$
$$x_{10} = -88.3523923305876$$
$$x_{44} = -83.6500809710862$$
$$x_{43} = -82.0688317633173$$
$$x_{17} = -80.5086903111165$$
$$x_{26} = -77.367316054284$$
$$x_{65} = -75.7852089599415$$
$$x_{85} = -74.2259602830878$$
$$x_{13} = -69.501507035948$$
$$x_{49} = -67.9433144530403$$
$$x_{27} = -66.3596193779403$$
$$x_{22} = -61.6607785557254$$
$$x_{31} = -60.0757514718742$$
$$x_{59} = -58.5195628953145$$
$$x_{18} = -55.3783900020515$$
$$x_{15} = -53.7917240434184$$
$$x_{87} = -52.2372675901086$$
$$x_{74} = -47.5074737578972$$
$$x_{41} = -45.9552156096449$$
$$x_{88} = -44.3652354315924$$
$$x_{6} = -41.2228986266421$$
$$x_{53} = -39.6735225187304$$
$$x_{60} = -38.0804389435651$$
$$x_{69} = -33.3923908048991$$
$$x_{34} = -31.7950050422583$$
$$x_{78} = -30.2521447011736$$
$$x_{19} = -25.5084617903844$$
$$x_{29} = -23.9727106489827$$
$$x_{84} = -22.3644811896294$$
$$x_{48} = -19.2197177647678$$
$$x_{14} = -17.6959380645662$$
$$x_{32} = -16.0737101108477$$
$$x_{20} = -14.5596242389401$$
$$x_{50} = -11.4262062008546$$
$$x_{86} = -9.77311232132626$$
$$x_{58} = -8.29895266969519$$
$$x_{72} = -6.61018866689758$$
$$x_{76} = -3.40573797678636$$
$$x_{24} = -2.16887082755251$$
$$x_{28} = 1.48855960517723$$
$$x_{4} = 2.57756789869373$$
$$x_{5} = 4.41833218870582$$
$$x_{33} = 5.81575314037901$$
$$x_{71} = 7.51899978446191$$
$$x_{35} = 10.6436031352424$$
$$x_{39} = 12.137966959545$$
$$x_{62} = 13.7759211770063$$
$$x_{40} = 18.4333574770289$$
$$x_{82} = 20.0492573318314$$
$$x_{45} = 21.578377129864$$
$$x_{73} = 24.7225236560155$$
$$x_{80} = 26.3272887370679$$
$$x_{67} = 27.8660932450522$$
$$x_{63} = 32.6073049652536$$
$$x_{52} = 34.1521398153172$$
$$x_{1} = 35.7477306320813$$
$$x_{55} = 40.4372963456374$$
$$x_{83} = 42.029105157638$$
$$x_{47} = 43.5796612954408$$
$$x_{16} = 46.7219223213739$$
$$x_{42} = 48.3109505632436$$
$$x_{61} = 49.8640990821956$$
$$x_{37} = 54.5931043261049$$
$$x_{68} = 56.1482564243358$$
$$x_{75} = 57.7342653899289$$
$$x_{38} = 62.4322180425498$$
$$x_{30} = 64.0167145801539$$
$$x_{8} = 65.574143045524$$
$$x_{36} = 68.7160376509593$$
$$x_{77} = 70.2992953395131$$
$$x_{79} = 71.8579058472682$$
$$x_{57} = 76.5819752896018$$
$$x_{46} = 78.1415757577305$$
$$x_{64} = 79.7233451333755$$
$$x_{70} = 84.4251735228072$$
$$x_{51} = 86.0061336494404$$
$$x_{12} = 87.5669502052005$$
$$x_{2} = 90.7087141383207$$
$$x_{3} = 92.2889761903236$$
$$x_{54} = 93.850466602772$$
$$x_{9} = 98.5718624260433$$
$$x_{66} = 100.133941444321$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} \leq x_{56}$$
For example, let's take the point
$$x_{0} = x_{56} - \frac{1}{10}$$
=
$$-104.061095462942 + - \frac{1}{10}$$
=
$$-104.161095462942$$
substitute to the expression
$$x \cos{\left(2 x - \frac{\pi}{4} \right)} \leq - \frac{\sqrt{3}}{2}$$
$$\left(-104.161095462942\right) \cos{\left(\left(-104.161095462942\right) 2 - \frac{\pi}{4} \right)} \leq - \frac{\sqrt{3}}{2}$$
___
/ pi\ -\/ 3
-104.161095462942*cos|208.322190925885 + --| <= -------
\ 4 / 2
but
___
/ pi\ -\/ 3
-104.161095462942*cos|208.322190925885 + --| >= -------
\ 4 / 2
Then
$$x \leq -104.061095462942$$
no execute
one of the solutions of our inequality is:
$$x \geq -104.061095462942 \wedge x \leq -99.3572258649787$$
_____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____
/ \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \
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x56 x23 x25 x21 x7 x11 x81 x10 x44 x43 x17 x26 x65 x85 x13 x49 x27 x22 x31 x59 x18 x15 x87 x74 x41 x88 x6 x53 x60 x69 x34 x78 x19 x29 x84 x48 x14 x32 x20 x50 x86 x58 x72 x76 x24 x28 x4 x5 x33 x71 x35 x39 x62 x40 x82 x45 x73 x80 x67 x63 x52 x1 x55 x83 x47 x16 x42 x61 x37 x68 x75 x38 x30 x8 x36 x77 x79 x57 x46 x64 x70 x51 x12 x2 x3 x54 x9 x66
Other solutions will get with the changeover to the next point
etc.
The answer:
$$x \geq -104.061095462942 \wedge x \leq -99.3572258649787$$
$$x \geq -97.7776427399657 \wedge x \leq -96.2157755104838$$
$$x \geq -94.6359030607806 \wedge x \leq -91.4941532834635$$
$$x \geq -89.9329046251528 \wedge x \leq -88.3523923305876$$
$$x \geq -83.6500809710862 \wedge x \leq -82.0688317633173$$
$$x \geq -80.5086903111165 \wedge x \leq -77.367316054284$$
$$x \geq -75.7852089599415 \wedge x \leq -74.2259602830878$$
$$x \geq -69.501507035948 \wedge x \leq -67.9433144530403$$
$$x \geq -66.3596193779403 \wedge x \leq -61.6607785557254$$
$$x \geq -60.0757514718742 \wedge x \leq -58.5195628953145$$
$$x \geq -55.3783900020515 \wedge x \leq -53.7917240434184$$
$$x \geq -52.2372675901086 \wedge x \leq -47.5074737578972$$
$$x \geq -45.9552156096449 \wedge x \leq -44.3652354315924$$
$$x \geq -41.2228986266421 \wedge x \leq -39.6735225187304$$
$$x \geq -38.0804389435651 \wedge x \leq -33.3923908048991$$
$$x \geq -31.7950050422583 \wedge x \leq -30.2521447011736$$
$$x \geq -25.5084617903844 \wedge x \leq -23.9727106489827$$
$$x \geq -22.3644811896294 \wedge x \leq -19.2197177647678$$
$$x \geq -17.6959380645662 \wedge x \leq -16.0737101108477$$
$$x \geq -14.5596242389401 \wedge x \leq -11.4262062008546$$
$$x \geq -9.77311232132626 \wedge x \leq -8.29895266969519$$
$$x \geq -6.61018866689758 \wedge x \leq -3.40573797678636$$
$$x \geq -2.16887082755251 \wedge x \leq 1.48855960517723$$
$$x \geq 2.57756789869373 \wedge x \leq 4.41833218870582$$
$$x \geq 5.81575314037901 \wedge x \leq 7.51899978446191$$
$$x \geq 10.6436031352424 \wedge x \leq 12.137966959545$$
$$x \geq 13.7759211770063 \wedge x \leq 18.4333574770289$$
$$x \geq 20.0492573318314 \wedge x \leq 21.578377129864$$
$$x \geq 24.7225236560155 \wedge x \leq 26.3272887370679$$
$$x \geq 27.8660932450522 \wedge x \leq 32.6073049652536$$
$$x \geq 34.1521398153172 \wedge x \leq 35.7477306320813$$
$$x \geq 40.4372963456374 \wedge x \leq 42.029105157638$$
$$x \geq 43.5796612954408 \wedge x \leq 46.7219223213739$$
$$x \geq 48.3109505632436 \wedge x \leq 49.8640990821956$$
$$x \geq 54.5931043261049 \wedge x \leq 56.1482564243358$$
$$x \geq 57.7342653899289 \wedge x \leq 62.4322180425498$$
$$x \geq 64.0167145801539 \wedge x \leq 65.574143045524$$
$$x \geq 68.7160376509593 \wedge x \leq 70.2992953395131$$
$$x \geq 71.8579058472682 \wedge x \leq 76.5819752896018$$
$$x \geq 78.1415757577305 \wedge x \leq 79.7233451333755$$
$$x \geq 84.4251735228072 \wedge x \leq 86.0061336494404$$
$$x \geq 87.5669502052005 \wedge x \leq 90.7087141383207$$
$$x \geq 92.2889761903236 \wedge x \leq 93.850466602772$$
$$x \geq 98.5718624260433 \wedge x \leq 100.133941444321$$