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D( x )
12*x+21 = 0
4*x+7 = 0
12*x+21 = 0
12*x+21 = 0
12*x+21 = 0 // - 21
12*x = -21 // : 12
x = -21/12
x = -7/4
4*x+7 = 0
4*x+7 = 0
4*x+7 = 0 // - 7
4*x = -7 // : 4
x = -7/4
x in (-oo:-7/4) U (-7/4:+oo)
2/(4*x+7)+x/3 = 6/(12*x+21) // - 6/(12*x+21)
2/(4*x+7)-(6/(12*x+21))+x/3 = 0
2/(4*x+7)-6*(12*x+21)^-1+x/3 = 0
2/(4*x+7)-6/(12*x+21)+x/3 = 0
(2*3*(12*x+21))/(3*(4*x+7)*(12*x+21))+(-6*3*(4*x+7))/(3*(4*x+7)*(12*x+21))+(x*(4*x+7)*(12*x+21))/(3*(4*x+7)*(12*x+21)) = 0
2*3*(12*x+21)-6*3*(4*x+7)+x*(4*x+7)*(12*x+21) = 0
48*x^3+168*x^2+147*x = 0
48*x^3+168*x^2+147*x = 0
3*x*(16*x^2+56*x+49) = 0
16*x^2+56*x+49 = 0
DELTA = 56^2-(4*16*49)
DELTA = 0
x = -56/(2*16)
x = -7/4 or x = -7/4
3*x*(x+7/4)^2 = 0
(3*x*(x+7/4)^2)/(3*(4*x+7)*(12*x+21)) = 0
(3*x*(x+7/4)^2)/(3*(4*x+7)*(12*x+21)) = 0 // * 3*(4*x+7)*(12*x+21)
3*x*(x+7/4)^2 = 0
( 3*x )
3*x = 0 // : 3
x = 0
( x+7/4 )
x+7/4 = 0 // - 7/4
x = -7/4
x in { -7/4}
x = 0
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