We know that cos(2x)=1−2sin2(x) . Now equation looks like
1−2sin2(x)+0.5sin(x)=0
Multiply the whole equation by -2 (just for convenience):
4sin2(x)−sin(x)−2=0
Now we have a standard quadratic equation for sin(x)
sin(x)=1/8(1±1+32)=1/8(1±33)
For the first root (sinx=1/8(1+33) ) the solution isx=(−1)karcsin(1/8(1+33))+πk≈(−1)k+πk
k =0: x1=1rad≈57°
k = 1: x2=−1+π=2.14rad≈123°
The same for the second root:
x=(−1)karcsin(1/8(1−33))+πk≈(−1)k∗(−0.635)+πk
k=1:x3=(0.635+π)rad≈3.78rad≈216°
k=2:x4=(−0.635+2π)rad≈5.645rad≈323°
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