(3x+10y)-(6x+3y)+(6y-8x) =
3x+10y-6x-3y+6y-8x =
13y-11x
X^2+15x>0
x(x+15)>0
x=0 или x+15=0
х=-15
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-15 0
(-∞.-15) U (0,+<span>∞)</span>
Cosα+cos2α+cos3α= (cosα +cos3α)+cos2α=
=2cos2α*cosα+cos2α=cos2α(2cosα+1)=
=2cos2α(cosα+1/2)=2cos2α(cosα+cosπ/3)=
=2cos2α*2cos(π/6+α/2)cos(π/6-α/2)=
=4cos2α*cos(π/6+α/2)cos(π/6-α/2)