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D( x )
7*x+21 = 0
x+3 = 0
7*x+21 = 0
7*x+21 = 0
7*x+21 = 0 // - 21
7*x = -21 // : 7
x = -21/7
x = -3
x+3 = 0
x+3 = 0
x+3 = 0 // - 3
x = -3
x in (-oo:-3) U (-3:+oo)
3/(7*x+21) = 4/(x+3)-(5/7) // - 4/(x+3)-(5/7)
3/(7*x+21)-(4/(x+3))+5/7 = 0
3/(7*x+21)-4*(x+3)^-1+5/7 = 0
3/(7*x+21)-4/(x+3)+5/7 = 0
(3*7*(x+3))/(7*(7*x+21)*(x+3))+(-4*7*(7*x+21))/(7*(7*x+21)*(x+3))+(5*(7*x+21)*(x+3))/(7*(7*x+21)*(x+3)) = 0
3*7*(x+3)-4*7*(7*x+21)+5*(7*x+21)*(x+3) = 0
35*x^2-175*x+210*x-525+315 = 0
35*x^2+35*x-210 = 0
35*x^2+35*x-210 = 0
35*(x^2+x-6) = 0
x^2+x-6 = 0
DELTA = 1^2-(-6*1*4)
DELTA = 25
DELTA > 0
x = (25^(1/2)-1)/(1*2) or x = (-25^(1/2)-1)/(1*2)
x = 2 or x = -3
35*(x+3)*(x-2) = 0
(35*(x+3)*(x-2))/(7*(7*x+21)*(x+3)) = 0
(35*(x+3)*(x-2))/(7*(7*x+21)*(x+3)) = 0 // * 7*(7*x+21)*(x+3)
35*(x+3)*(x-2) = 0
( x+3 )
x+3 = 0 // - 3
x = -3
( x-2 )
x-2 = 0 // + 2
x = 2
x in { -3}
x = 2
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