it has to be in degrees
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2cos²(x)+3cos(x)-2=(2cos(x)-1)(cos(x)+2)=0,

Therefore 2cos(x)-1=0, cos(x)=½, x=60°, 300°, 420°, 660°, ...

Note that cos(x)+2=0 has no solution for x, because cos(x) cannot be less than -1 or greater than 1.

by Top Rated User (813k points)

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