2(1-cos^2x-cos^2x)=1
2-4cos^2x=1
-4cos^2x=-1
cos^2x=1/4
1) cosx=1/2
x=+-pi/3+2pi*n
2) cosx=-1/2
x=+-2pi/3+2pi*n
Ответ: x1=pi/3+2pi*n; x2=-pi/3+2pi*n; x3=2pi/3+2pi*n; x4=-2pi/3+2pi*n
2x+5y=-11
x-3y=11
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2x+5y=-11
x=3y+11
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2(3y+11)+5y=-11
x=3y+11
----------------------
6y+22+5y=-11
x=3y+11
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11y=-33
x=3y+11
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y=-3
x=-3.3+11
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y=-3
x=2
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/x,y/=(2,-3)
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xy=-6
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Otvet: -6