Cosα+cos2α+cos6α+cos7α=(cosα+cos7α)+(cos2α+cos6α)=
=2·cos<span>(α+7α)/2</span>·cos((α-7α)/2+2·cos(2α+6α)/2·cos(2α-6α)/2=
=2cos4α·cos(-3α)+2·cos4α·cos(-2α)=
=2·cos4α·(cos3α+cos2α)=2cos4α·2·cos5α/2·cosα/2=
=4cos4α·cos5α/2·cosα/2;
3*(6a-3)=6a+4; 18a-9=6a+4; 18a-6a=4+9; 12a=13; a=13/12.
Вторая четверть x<=0, y>=0
третья x<=0, y<=0
четвертая x>=0, y<=0