cos0+cospi/7+cos2pi/7+cos3pi/7+cos4pi/7+cos5pi/7+cos6pi/7=
=1+2сosП/2сos5П/14+2cosП/2cos3П/14+2cosП/2cosП/2cosП/14=1
Имеет.
<span>4x-6y=2 </span>
<span>3y-2x=1</span>
<span>
</span>
<span><span>4x-6y=2 </span></span>
<span><span>x=-0.5+1.5у</span></span>
<span><span>
</span></span>
<span><span><span>4(-0.5+1.5у)-6y=2 и решаете</span></span></span>