<span>1) cos4x = -2cos²x</span>
<span>2cos²x - 1 = - 1 - cos2x</span>
<span>2cos²x - 1 + 1 + cos2x = 0</span>
<span>2cos²x + cos2x = 0</span>
<span>2cos²x +1 -2sin²x=0</span>
<span>2cos²x -2sin²x= - 1</span>
<span>2(cos²x - sin²x) = -1</span>
<span>cos²x - sin²x = -1/2</span>
<span>cos2x = -1/2</span>
<span>2x = ±2π/ 3 +2 πn, n ∈ Z</span>
<span>x = ±π/ 3 + πn, n ∈ Z</span>
<span>2) <span>[0;180⁰]</span></span>
<span><span>x = -π/ 3 + πn, n ∈ Z</span></span>