X + 3y = 1
3y = 1 - x
y = ( 1 - x ) : 3
y = 1/3 - 1/3x
x = 1 - 3y
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2x - 3y = 1
3y = 2x - 1
y = ( 2x - 1 ) : 3
y = 2/3x - 1/3
2x = 3y + 1
x = ( 3y + 1 ) : 2
x = 1,5y + 0,5
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4y + 4x - 1 = 0
4y = 1 - 4x
y = ( 1 - 4x ) : 4
y = 0,25 - x
4x = 1 - 4y
x = ( 1 - 4y ) : 4
x = 0,25 - y