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D( x )
(0.8-x)^2 = 0
(0.8-x)^2 = 0
(0.8-x)^2 = 0
0.8-x = 0 // - 0.8
-x = -0.8 // * -1
x = 0.8
x in (-oo:0.8) U (0.8:+oo)
((2*x)^2)/((0.8-x)^2) = 0
(4*x^2)/((0.8-x)^2) = 0
4*x^2 = 0 // : 4
x^2 = 0
x = 0
x = 0
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