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cos(t)≤-√2/2 inequation

A inequation with variable

The solution

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             ___ 
          -\/ 2  
cos(t) <= -------
             2   
$$\cos{\left(t \right)} \leq \frac{\left(-1\right) \sqrt{2}}{2}$$
cos(t) <= (-sqrt(2))/2
Detail solution
Given the inequality:
$$\cos{\left(t \right)} \leq \frac{\left(-1\right) \sqrt{2}}{2}$$
To solve this inequality, we must first solve the corresponding equation:
$$\cos{\left(t \right)} = \frac{\left(-1\right) \sqrt{2}}{2}$$
Solve:
$$x_{1} = -79.3252145031423$$
$$x_{2} = -66.7588438887831$$
$$x_{3} = -73.0420291959627$$
$$x_{4} = -85.6083998103219$$
$$x_{5} = -21.2057504117311$$
$$x_{6} = -10.2101761241668$$
$$x_{7} = -335.36501577071$$
$$x_{8} = 41.6261026600648$$
$$x_{9} = -29.0597320457056$$
$$x_{10} = 90.3207887907066$$
$$x_{11} = -35.3429173528852$$
$$x_{12} = 21.2057504117311$$
$$x_{13} = 2.35619449019234$$
$$x_{14} = 8.63937979737193$$
$$x_{15} = -60.4756585816035$$
$$x_{16} = -52.621676947629$$
$$x_{17} = 255.254403104171$$
$$x_{18} = 79.3252145031423$$
$$x_{19} = 3.92699081698724$$
$$x_{20} = 35.3429173528852$$
$$x_{21} = -46.3384916404494$$
$$x_{22} = 60.4756585816035$$
$$x_{23} = -27.4889357189107$$
$$x_{24} = -2.35619449019234$$
$$x_{25} = 29.0597320457056$$
$$x_{26} = 71.4712328691678$$
$$x_{27} = -90.3207887907066$$
$$x_{28} = 39349.2333843756$$
$$x_{29} = 14.9225651045515$$
$$x_{30} = 47.9092879672443$$
$$x_{31} = -77.7544181763474$$
$$x_{32} = 33.7721210260903$$
$$x_{33} = 73.0420291959627$$
$$x_{34} = 77.7544181763474$$
$$x_{35} = -54.1924732744239$$
$$x_{36} = 54.1924732744239$$
$$x_{37} = 46.3384916404494$$
$$x_{38} = -47.9092879672443$$
$$x_{39} = -65.1880475619882$$
$$x_{40} = 2584.745355741$$
$$x_{41} = 96.6039740978861$$
$$x_{42} = 66.7588438887831$$
$$x_{43} = -96.6039740978861$$
$$x_{44} = 65.1880475619882$$
$$x_{45} = 104.457955731861$$
$$x_{46} = 85.6083998103219$$
$$x_{47} = 16.4933614313464$$
$$x_{48} = -71.4712328691678$$
$$x_{49} = -8.63937979737193$$
$$x_{50} = -3.92699081698724$$
$$x_{51} = 27.4889357189107$$
$$x_{52} = 98.174770424681$$
$$x_{53} = 52.621676947629$$
$$x_{54} = -14.9225651045515$$
$$x_{55} = -40.0553063332699$$
$$x_{56} = 22.776546738526$$
$$x_{57} = -41.6261026600648$$
$$x_{58} = -16.4933614313464$$
$$x_{59} = 40.0553063332699$$
$$x_{60} = 58.9048622548086$$
$$x_{61} = -58.9048622548086$$
$$x_{62} = -84.037603483527$$
$$x_{63} = 91.8915851175014$$
$$x_{64} = -33.7721210260903$$
$$x_{65} = 10.2101761241668$$
$$x_{66} = -22.776546738526$$
$$x_{67} = -98.174770424681$$
$$x_{68} = 84.037603483527$$
$$x_{69} = -91.8915851175014$$
$$x_{1} = -79.3252145031423$$
$$x_{2} = -66.7588438887831$$
$$x_{3} = -73.0420291959627$$
$$x_{4} = -85.6083998103219$$
$$x_{5} = -21.2057504117311$$
$$x_{6} = -10.2101761241668$$
$$x_{7} = -335.36501577071$$
$$x_{8} = 41.6261026600648$$
$$x_{9} = -29.0597320457056$$
$$x_{10} = 90.3207887907066$$
$$x_{11} = -35.3429173528852$$
$$x_{12} = 21.2057504117311$$
$$x_{13} = 2.35619449019234$$
$$x_{14} = 8.63937979737193$$
$$x_{15} = -60.4756585816035$$
$$x_{16} = -52.621676947629$$
$$x_{17} = 255.254403104171$$
$$x_{18} = 79.3252145031423$$
$$x_{19} = 3.92699081698724$$
$$x_{20} = 35.3429173528852$$
$$x_{21} = -46.3384916404494$$
$$x_{22} = 60.4756585816035$$
$$x_{23} = -27.4889357189107$$
$$x_{24} = -2.35619449019234$$
$$x_{25} = 29.0597320457056$$
$$x_{26} = 71.4712328691678$$
$$x_{27} = -90.3207887907066$$
$$x_{28} = 39349.2333843756$$
$$x_{29} = 14.9225651045515$$
$$x_{30} = 47.9092879672443$$
$$x_{31} = -77.7544181763474$$
$$x_{32} = 33.7721210260903$$
$$x_{33} = 73.0420291959627$$
$$x_{34} = 77.7544181763474$$
$$x_{35} = -54.1924732744239$$
$$x_{36} = 54.1924732744239$$
$$x_{37} = 46.3384916404494$$
$$x_{38} = -47.9092879672443$$
$$x_{39} = -65.1880475619882$$
$$x_{40} = 2584.745355741$$
$$x_{41} = 96.6039740978861$$
$$x_{42} = 66.7588438887831$$
$$x_{43} = -96.6039740978861$$
$$x_{44} = 65.1880475619882$$
$$x_{45} = 104.457955731861$$
$$x_{46} = 85.6083998103219$$
$$x_{47} = 16.4933614313464$$
$$x_{48} = -71.4712328691678$$
$$x_{49} = -8.63937979737193$$
$$x_{50} = -3.92699081698724$$
$$x_{51} = 27.4889357189107$$
$$x_{52} = 98.174770424681$$
$$x_{53} = 52.621676947629$$
$$x_{54} = -14.9225651045515$$
$$x_{55} = -40.0553063332699$$
$$x_{56} = 22.776546738526$$
$$x_{57} = -41.6261026600648$$
$$x_{58} = -16.4933614313464$$
$$x_{59} = 40.0553063332699$$
$$x_{60} = 58.9048622548086$$
$$x_{61} = -58.9048622548086$$
$$x_{62} = -84.037603483527$$
$$x_{63} = 91.8915851175014$$
$$x_{64} = -33.7721210260903$$
$$x_{65} = 10.2101761241668$$
$$x_{66} = -22.776546738526$$
$$x_{67} = -98.174770424681$$
$$x_{68} = 84.037603483527$$
$$x_{69} = -91.8915851175014$$
This roots
$$x_{7} = -335.36501577071$$
$$x_{67} = -98.174770424681$$
$$x_{43} = -96.6039740978861$$
$$x_{69} = -91.8915851175014$$
$$x_{27} = -90.3207887907066$$
$$x_{4} = -85.6083998103219$$
$$x_{62} = -84.037603483527$$
$$x_{1} = -79.3252145031423$$
$$x_{31} = -77.7544181763474$$
$$x_{3} = -73.0420291959627$$
$$x_{48} = -71.4712328691678$$
$$x_{2} = -66.7588438887831$$
$$x_{39} = -65.1880475619882$$
$$x_{15} = -60.4756585816035$$
$$x_{61} = -58.9048622548086$$
$$x_{35} = -54.1924732744239$$
$$x_{16} = -52.621676947629$$
$$x_{38} = -47.9092879672443$$
$$x_{21} = -46.3384916404494$$
$$x_{57} = -41.6261026600648$$
$$x_{55} = -40.0553063332699$$
$$x_{11} = -35.3429173528852$$
$$x_{64} = -33.7721210260903$$
$$x_{9} = -29.0597320457056$$
$$x_{23} = -27.4889357189107$$
$$x_{66} = -22.776546738526$$
$$x_{5} = -21.2057504117311$$
$$x_{58} = -16.4933614313464$$
$$x_{54} = -14.9225651045515$$
$$x_{6} = -10.2101761241668$$
$$x_{49} = -8.63937979737193$$
$$x_{50} = -3.92699081698724$$
$$x_{24} = -2.35619449019234$$
$$x_{13} = 2.35619449019234$$
$$x_{19} = 3.92699081698724$$
$$x_{14} = 8.63937979737193$$
$$x_{65} = 10.2101761241668$$
$$x_{29} = 14.9225651045515$$
$$x_{47} = 16.4933614313464$$
$$x_{12} = 21.2057504117311$$
$$x_{56} = 22.776546738526$$
$$x_{51} = 27.4889357189107$$
$$x_{25} = 29.0597320457056$$
$$x_{32} = 33.7721210260903$$
$$x_{20} = 35.3429173528852$$
$$x_{59} = 40.0553063332699$$
$$x_{8} = 41.6261026600648$$
$$x_{37} = 46.3384916404494$$
$$x_{30} = 47.9092879672443$$
$$x_{53} = 52.621676947629$$
$$x_{36} = 54.1924732744239$$
$$x_{60} = 58.9048622548086$$
$$x_{22} = 60.4756585816035$$
$$x_{44} = 65.1880475619882$$
$$x_{42} = 66.7588438887831$$
$$x_{26} = 71.4712328691678$$
$$x_{33} = 73.0420291959627$$
$$x_{34} = 77.7544181763474$$
$$x_{18} = 79.3252145031423$$
$$x_{68} = 84.037603483527$$
$$x_{46} = 85.6083998103219$$
$$x_{10} = 90.3207887907066$$
$$x_{63} = 91.8915851175014$$
$$x_{41} = 96.6039740978861$$
$$x_{52} = 98.174770424681$$
$$x_{45} = 104.457955731861$$
$$x_{17} = 255.254403104171$$
$$x_{40} = 2584.745355741$$
$$x_{28} = 39349.2333843756$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} \leq x_{7}$$
For example, let's take the point
$$x_{0} = x_{7} - \frac{1}{10}$$
=
$$-335.36501577071 + - \frac{1}{10}$$
=
$$-335.46501577071$$
substitute to the expression
$$\cos{\left(t \right)} \leq \frac{\left(-1\right) \sqrt{2}}{2}$$
$$\cos{\left(t \right)} \leq \frac{\left(-1\right) \sqrt{2}}{2}$$
             ___ 
          -\/ 2  
cos(t) <= -------
             2   
          

Then
$$x \leq -335.36501577071$$
no execute
one of the solutions of our inequality is:
$$x \geq -335.36501577071 \wedge x \leq -98.174770424681$$
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-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------•-------
       x7      x67      x43      x69      x27      x4      x62      x1      x31      x3      x48      x2      x39      x15      x61      x35      x16      x38      x21      x57      x55      x11      x64      x9      x23      x66      x5      x58      x54      x6      x49      x50      x24      x13      x19      x14      x65      x29      x47      x12      x56      x51      x25      x32      x20      x59      x8      x37      x30      x53      x36      x60      x22      x44      x42      x26      x33      x34      x18      x68      x46      x10      x63      x41      x52      x45      x17      x40      x28

Other solutions will get with the changeover to the next point
etc.
The answer:
$$x \geq -335.36501577071 \wedge x \leq -98.174770424681$$
$$x \geq -96.6039740978861 \wedge x \leq -91.8915851175014$$
$$x \geq -90.3207887907066 \wedge x \leq -85.6083998103219$$
$$x \geq -84.037603483527 \wedge x \leq -79.3252145031423$$
$$x \geq -77.7544181763474 \wedge x \leq -73.0420291959627$$
$$x \geq -71.4712328691678 \wedge x \leq -66.7588438887831$$
$$x \geq -65.1880475619882 \wedge x \leq -60.4756585816035$$
$$x \geq -58.9048622548086 \wedge x \leq -54.1924732744239$$
$$x \geq -52.621676947629 \wedge x \leq -47.9092879672443$$
$$x \geq -46.3384916404494 \wedge x \leq -41.6261026600648$$
$$x \geq -40.0553063332699 \wedge x \leq -35.3429173528852$$
$$x \geq -33.7721210260903 \wedge x \leq -29.0597320457056$$
$$x \geq -27.4889357189107 \wedge x \leq -22.776546738526$$
$$x \geq -21.2057504117311 \wedge x \leq -16.4933614313464$$
$$x \geq -14.9225651045515 \wedge x \leq -10.2101761241668$$
$$x \geq -8.63937979737193 \wedge x \leq -3.92699081698724$$
$$x \geq -2.35619449019234 \wedge x \leq 2.35619449019234$$
$$x \geq 3.92699081698724 \wedge x \leq 8.63937979737193$$
$$x \geq 10.2101761241668 \wedge x \leq 14.9225651045515$$
$$x \geq 16.4933614313464 \wedge x \leq 21.2057504117311$$
$$x \geq 22.776546738526 \wedge x \leq 27.4889357189107$$
$$x \geq 29.0597320457056 \wedge x \leq 33.7721210260903$$
$$x \geq 35.3429173528852 \wedge x \leq 40.0553063332699$$
$$x \geq 41.6261026600648 \wedge x \leq 46.3384916404494$$
$$x \geq 47.9092879672443 \wedge x \leq 52.621676947629$$
$$x \geq 54.1924732744239 \wedge x \leq 58.9048622548086$$
$$x \geq 60.4756585816035 \wedge x \leq 65.1880475619882$$
$$x \geq 66.7588438887831 \wedge x \leq 71.4712328691678$$
$$x \geq 73.0420291959627 \wedge x \leq 77.7544181763474$$
$$x \geq 79.3252145031423 \wedge x \leq 84.037603483527$$
$$x \geq 85.6083998103219 \wedge x \leq 90.3207887907066$$
$$x \geq 91.8915851175014 \wedge x \leq 96.6039740978861$$
$$x \geq 98.174770424681 \wedge x \leq 104.457955731861$$
$$x \geq 255.254403104171 \wedge x \leq 2584.745355741$$
$$x \geq 39349.2333843756$$
Rapid solution 2 [src]
 3*pi  5*pi 
[----, ----]
  4     4   
$$x\ in\ \left[\frac{3 \pi}{4}, \frac{5 \pi}{4}\right]$$
x in Interval(3*pi/4, 5*pi/4)
Rapid solution [src]
   /3*pi            5*pi\
And|---- <= x, x <= ----|
   \ 4               4  /
$$\frac{3 \pi}{4} \leq x \wedge x \leq \frac{5 \pi}{4}$$
(3*pi/4 <= x)∧(x <= 5*pi/4)