Ответ:

Пошаговое объяснение:

1)  4х+у=8;

4x=8-y;

x=(8-y)/4;

x=2-y/4.

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y=8-4x.

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2)  5x+2y=1;

5x=1-2y;

x=(1-2y)/5;

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2y=1-5x;

y=(1-5x)/2.

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3)   15x+3y=3;

15x=3-3y;

x=(3-3y)/15=0.2(1-y);

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3y=3-15x;

y=1-5x;

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4)  3x+7y=42;

3x=42-7y;

x=14  -  2  1/3y;

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7y=42-3x;

y=6+3/7x.

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5)  x+3y=12;

x=12-3y;

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3y=12-x;

y=4-1/3x.

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6)  4x+3y=-1;

4x=-1-3y;

x= -(1+3y)/4;

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3y=-1-4x;

y=  -(1+4x)/3;

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7)   20x-7y=-5;

20x=-5+7y;

x=(7y-5)/20;

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-7y=-5-20x;

y=(5+20x)/7.