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sin2x+16cos^2x=4 equation

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Numerical solution:

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The solution

You have entered [src]
                 2       
sin(2*x) + 16*cos (x) = 4
$$\sin{\left(2 x \right)} + 16 \cos^{2}{\left(x \right)} = 4$$
The graph
Rapid solution [src]
             /log(5)      /  ___\\          
x1 = -pi + I*|------ - log\\/ 5 /| + atan(2)
             \  2                /          
$$x_{1} = - \pi + \operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)$$
       /log(5)      /  ___\\          
x2 = I*|------ - log\\/ 5 /| + atan(2)
       \  2                /          
$$x_{2} = \operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)$$
                        /log(13)      /  ____\\
x3 = pi - atan(3/2) + I*|------- - log\\/ 13 /|
                        \   2                 /
$$x_{3} = - \operatorname{atan}{\left(\frac{3}{2} \right)} + \pi + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)$$
                    /log(13)      /  ____\\
x4 = -atan(3/2) + I*|------- - log\\/ 13 /|
                    \   2                 /
$$x_{4} = - \operatorname{atan}{\left(\frac{3}{2} \right)} + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)$$
x4 = -atan(3/2) + i*(-log(sqrt(13)) + log(13)/2)
Sum and product of roots [src]
sum
        /log(5)      /  ___\\               /log(5)      /  ___\\                                /log(13)      /  ____\\                  /log(13)      /  ____\\
-pi + I*|------ - log\\/ 5 /| + atan(2) + I*|------ - log\\/ 5 /| + atan(2) + pi - atan(3/2) + I*|------- - log\\/ 13 /| + -atan(3/2) + I*|------- - log\\/ 13 /|
        \  2                /               \  2                /                                \   2                 /                  \   2                 /
$$\left(\left(\left(- \pi + \operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)\right) + \left(\operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)\right)\right) + \left(- \operatorname{atan}{\left(\frac{3}{2} \right)} + \pi + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)\right)\right) + \left(- \operatorname{atan}{\left(\frac{3}{2} \right)} + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)\right)$$
=
                               /log(5)      /  ___\\       /log(13)      /  ____\\
-2*atan(3/2) + 2*atan(2) + 2*I*|------ - log\\/ 5 /| + 2*I*|------- - log\\/ 13 /|
                               \  2                /       \   2                 /
$$- 2 \operatorname{atan}{\left(\frac{3}{2} \right)} + 2 \operatorname{atan}{\left(2 \right)} + 2 i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right) + 2 i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)$$
product
/        /log(5)      /  ___\\          \ /  /log(5)      /  ___\\          \ /                   /log(13)      /  ____\\\ /               /log(13)      /  ____\\\
|-pi + I*|------ - log\\/ 5 /| + atan(2)|*|I*|------ - log\\/ 5 /| + atan(2)|*|pi - atan(3/2) + I*|------- - log\\/ 13 /||*|-atan(3/2) + I*|------- - log\\/ 13 /||
\        \  2                /          / \  \  2                /          / \                   \   2                 // \               \   2                 //
$$\left(\operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)\right) \left(- \pi + \operatorname{atan}{\left(2 \right)} + i \left(- \log{\left(\sqrt{5} \right)} + \frac{\log{\left(5 \right)}}{2}\right)\right) \left(- \operatorname{atan}{\left(\frac{3}{2} \right)} + \pi + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)\right) \left(- \operatorname{atan}{\left(\frac{3}{2} \right)} + i \left(- \log{\left(\sqrt{13} \right)} + \frac{\log{\left(13 \right)}}{2}\right)\right)$$
=
/  2                                                \                  
\pi  + atan(2)*atan(3/2) - pi*atan(2) - pi*atan(3/2)/*atan(2)*atan(3/2)
$$\left(- \pi \operatorname{atan}{\left(2 \right)} - \pi \operatorname{atan}{\left(\frac{3}{2} \right)} + \operatorname{atan}{\left(\frac{3}{2} \right)} \operatorname{atan}{\left(2 \right)} + \pi^{2}\right) \operatorname{atan}{\left(\frac{3}{2} \right)} \operatorname{atan}{\left(2 \right)}$$
(pi^2 + atan(2)*atan(3/2) - pi*atan(2) - pi*atan(3/2))*atan(2)*atan(3/2)
Numerical answer [src]
x1 = 39.85791077342
x2 = 54.5142238288206
x3 = -2.0344439357957
x4 = -74.2910749683609
x5 = 33.5747254662404
x6 = -68.0078896611814
x7 = 24.149947505471
x8 = 46.1410960805996
x9 = 63.93900178959
x10 = -33.4503704716936
x11 = 60.7974091360002
x12 = 68.1322446557281
x13 = -54.3898688342738
x14 = 32.523075253692
x15 = 38.8062605608716
x16 = 76.5053724039491
x17 = 99.5481711916261
x18 = 74.4154299629077
x19 = -82.664202716582
x20 = 8.44198423752205
x21 = -32.3987202591453
x22 = 52.4242813877792
x23 = 27.2915401590608
x24 = 11.5835768911118
x25 = -48.1066835270942
x26 = 85.9301503647185
x27 = -77.4326676219507
x28 = 26.2398899465124
x29 = 70.2221870967695
x30 = 90.1233932308567
x31 = 48.231038521641
x32 = -63.8146467950432
x33 = -13.5491643376065
x34 = -76.3810174094024
x35 = -93.1406308898997
x36 = -41.8234982199146
x37 = -49.1583337396426
x38 = -39.7335557788732
x39 = 4.24874137138388
x40 = -8.31762924297529
x41 = -30.3087778181038
x42 = -4.12438637683712
x43 = 17.8667621982914
x44 = -79.5226100629922
x45 = 2.15879893034246
x46 = 35.6646679072818
x47 = -89.9990382363099
x48 = 30.4331328126506
x49 = -98.3721659845309
x50 = -17.7424072037447
x51 = -96.2822235434895
x52 = 57.6558164824104
x53 = 41.9478532144614
x54 = -85.8057953701718
x55 = -60.6730541414534
x56 = -55.4415190468222
x57 = 96.4065785380363
x58 = 13.6735193321533
x59 = 61.8490593485485
x60 = -36.5919631252834
x61 = 55.565874041369
x62 = -10.4075716840167
x63 = 101.638113632667
x64 = -61.7247043540018
x65 = -26.1155349519657
x66 = -19.8323496447861
x67 = -46.0167410860528
x68 = -35.5403129127351
x69 = -27.167185164514
x70 = 83.8402079236771
x71 = -11.4592218965651
x72 = -24.0255925109243
x73 = 16.8151119857431
x74 = -57.5314614878636
x75 = -92.0889806773513
x76 = 77.5570226164975
x77 = 19.9567046393328
x78 = 92.2133356718981
x79 = -5.1760365893855
x80 = 10.5319266785635
x81 = 98.4965209790777
x82 = -71.1494823147711
x83 = -70.0978321022228
x84 = -99.4238161970793
x85 = -52.2999263932324
x86 = 82.7885577111287
x87 = -83.7158529291303
x87 = -83.7158529291303