<span>cos3x=-1/2</span>
<span>3x=+-2pi/3+2pi n</span>
<span>x=+-2pi/9 +2/3 pi n</span>
<span>tg6x=-1</span>
<span>6x=-pi/4+pi n</span>
<span>x=-pi/24 +1/6pi n</span>
x^2-289=0
x^2=289
x1=17
x2=-17