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D( x )
7-x = 0
7-x = 0
7-x = 0
7-x = 0 // - 7
-x = -7 // * -1
x = 7
x in (-oo:7) U (7:+oo)
X/(7-x) = 2*x // - 2*x
X/(7-x)-(2*x) = 0
X/(7-x)-2*x = 0
X/(7-x)+(-2*x*(7-x))/(7-x) = 0
X-2*x*(7-x) = 0
X+2*x^2-14*x = 0
(X+2*x^2-14*x)/(7-x) = 0
(X+2*x^2-14*x)/(7-x) = 0 // * 7-x
X+2*x^2-14*x = 0
X+2*x^2-14*x = 0
DELTA = (-14)^2-(2*4*X)
DELTA = 196-8*X
196-8*X = 0
196-8*X = 0 // - 196
-8*X = -196 // : -8
X = -196/(-8)
X = 49/2
DELTA = 0 <=> t_3 = 49/2
x = 14/(2*2) i X = 49/2
x = 7/2 i X = 49/2
( x = ((196-8*X)^(1/2)+14)/(2*2) or x = (14-(196-8*X)^(1/2))/(2*2) ) i X > 49/2
( x = ((196-8*X)^(1/2)+14)/4 or x = (14-(196-8*X)^(1/2))/4 ) i X > 49/2
X-49/2 > 0
X-49/2 > 0 // + 49/2
X > 49/2
x in { 7/2, ((196-8*X)^(1/2)+14)/4, (14-(196-8*X)^(1/2))/4 }
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