2000-5%=1900 руб с человека
6*1900=11400 руб
4)11400 - верный
-cosx+cosx/2=1
1-2cos²x/2+cosx/2-1=0
-2cos²x/2+cosx/2=0
cosx/2(1-2cosx/2)=0
cosx/2=0⇒x/2=π/2+πn⇒x=π+2πn
cosx/2=1/2⇒x/2=+-π/3+2πn⇒x=+-2π/3+4πn
Вот держи решай переписывай
2sin2x<1
sin2x<1/2
-<span>π</span>/6<2x<<span>π/6</span>
<span>-<span>π/12<x<<span>π/12</span></span></span>
<span><span><span>x<span>∈(-<span>π/12;<span>π/12)</span></span></span></span></span></span>