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D( x )
x^2+10*x+21 = 0
x+7 = 0
x+3 = 0
x^2+10*x+21 = 0
x^2+10*x+21 = 0
x^2+10*x+21 = 0
DELTA = 10^2-(1*4*21)
DELTA = 16
DELTA > 0
x = (16^(1/2)-10)/(1*2) or x = (-16^(1/2)-10)/(1*2)
x = -3 or x = -7
x+7 = 0
x+7 = 0
x+7 = 0 // - 7
x = -7
x+3 = 0
x+3 = 0
x+3 = 0 // - 3
x = -3
x in (-oo:-7) U (-7:-3) U (-3:+oo)
x/(x+3)+5/(x+7) = 13/(x^2+10*x+21) // - 13/(x^2+10*x+21)
x/(x+3)+5/(x+7)-(13/(x^2+10*x+21)) = 0
x/(x+3)+5/(x+7)-13*(x^2+10*x+21)^-1 = 0
x/(x+3)+5/(x+7)-13/(x^2+10*x+21) = 0
x^2+10*x+21 = 0
x^2+10*x+21 = 0
x^2+10*x+21 = 0
DELTA = 10^2-(1*4*21)
DELTA = 16
DELTA > 0
x = (16^(1/2)-10)/(1*2) or x = (-16^(1/2)-10)/(1*2)
x = -3 or x = -7
(x+7)*(x+3) = 0
x/(x+3)+5/(x+7)-13/((x+7)*(x+3)) = 0
(x*(x+7))/((x+3)*(x+7))+(5*(x+3))/((x+3)*(x+7))-13/((x+3)*(x+7)) = 0
x*(x+7)+5*(x+3)-13 = 0
x^2+12*x-13+15 = 0
x^2+12*x+2 = 0
x^2+12*x+2 = 0
x^2+12*x+2 = 0
DELTA = 12^2-(1*2*4)
DELTA = 136
DELTA > 0
x = (136^(1/2)-12)/(1*2) or x = (-136^(1/2)-12)/(1*2)
x = (2*34^(1/2)-12)/2 or x = (-2*34^(1/2)-12)/2
(x-((-2*34^(1/2)-12)/2))*(x-((2*34^(1/2)-12)/2)) = 0
((x-((-2*34^(1/2)-12)/2))*(x-((2*34^(1/2)-12)/2)))/((x+3)*(x+7)) = 0
((x-((-2*34^(1/2)-12)/2))*(x-((2*34^(1/2)-12)/2)))/((x+3)*(x+7)) = 0 // * (x+3)*(x+7)
(x-((-2*34^(1/2)-12)/2))*(x-((2*34^(1/2)-12)/2)) = 0
( x-((-2*34^(1/2)-12)/2) )
x-((-2*34^(1/2)-12)/2) = 0 // + (-2*34^(1/2)-12)/2
x = (-2*34^(1/2)-12)/2
( x-((2*34^(1/2)-12)/2) )
x-((2*34^(1/2)-12)/2) = 0 // + (2*34^(1/2)-12)/2
x = (2*34^(1/2)-12)/2
x in { (-2*34^(1/2)-12)/2, (2*34^(1/2)-12)/2 }
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