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D( x )
3*x+1 = 0
x-1 = 0
3*x+1 = 0
3*x+1 = 0
3*x+1 = 0 // - 1
3*x = -1 // : 3
x = -1/3
x-1 = 0
x-1 = 0
x-1 = 0 // + 1
x = 1
x in (-oo:-1/3) U (-1/3:1) U (1:+oo)
4/(x-1)+6/(3*x+1) = 15/(3*x+1) // - 15/(3*x+1)
4/(x-1)+6/(3*x+1)-(15/(3*x+1)) = 0
4/(x-1)+6/(3*x+1)-15*(3*x+1)^-1 = 0
4/(x-1)+6/(3*x+1)-15/(3*x+1) = 0
(4*(3*x+1))/((x-1)*(3*x+1))+(6*(x-1))/((x-1)*(3*x+1))+(-15*(x-1))/((x-1)*(3*x+1)) = 0
4*(3*x+1)+6*(x-1)-15*(x-1) = 0
18*x-15*x-2+15 = 0
3*x+13 = 0
(3*x+13)/((x-1)*(3*x+1)) = 0
(3*x+13)/((x-1)*(3*x+1)) = 0 // * (x-1)*(3*x+1)
3*x+13 = 0
3*x+13 = 0 // - 13
3*x = -13 // : 3
x = -13/3
x = -13/3
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