А) cos2x+1=√2cos(п/2) б) на промежутке [2п; 7п/2] решите подробно на

2сosx+1=0
cosx=-1/2
x=-2π/3+2πn U x=2π/3+2πk
2π≤-2π/3+2πn≤7π/2
12≤-4+12n≤21
16≤12n≤25
4/3≤n≤25/12
n=2⇒x=-2π/3+4π=10π/3
2π≤2π/3+2πk≤7π/2
12≤4+12k≤21
8≤12k≤17
2/3≤k≤17/12
k=1⇒x=2π/3+2π=8π/3