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D( x )
3*x-21 = 0
4*x^2 = 0
3*x-21 = 0
3*x-21 = 0
3*x-21 = 0 // + 21
3*x = 21 // : 3
x = 21/3
x = 7
4*x^2 = 0
4*x^2 = 0
4*x^2 = 0 // : 4
x^2 = 0
x = 0
x in (-oo:0) U (0:7) U (7:+oo)
((6*x-18)/(4*x^2))*((x^2-(6*x)-7)/(3*x-21)) = 0
((6*x-18)/(4*x^2))*((x^2-6*x-7)/(3*x-21)) = 0
((6*x-18)*(x^2-6*x-7))/(4*x^2*(3*x-21)) = 0
x^2-6*x-7 = 0
x^2-6*x-7 = 0
DELTA = (-6)^2-(-7*1*4)
DELTA = 64
DELTA > 0
x = (64^(1/2)+6)/(1*2) or x = (6-64^(1/2))/(1*2)
x = 7 or x = -1
(x+1)*(x-7) = 0
((6*x-18)*(x+1)*(x-7))/(4*x^2*(3*x-21)) = 0
( 6*x-18 )
6*x-18 = 0 // + 18
6*x = 18 // : 6
x = 18/6
x = 3
( x+1 )
x+1 = 0 // - 1
x = -1
( x-7 )
x-7 = 0 // + 7
x = 7
x in { 7}
x in { 3, -1 }
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