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D( x )
x < 0
x < 0
x in <0:+oo)
v = (1/3)*(3.5*x^4.2)^2 // - (1/3)*(3.5*x^4.2)^2
v-((1/3)*(3.5*x^4.2)^2) = 0
v+(-1/3)*(3.5*x^4.2)^2 = 0
-1/3*3.5^2*x^8.4 = -v // : -1/3*3.5^2
x^8.4 = 3*3.5^-2*v
x^8 = (3*3.5^-2*v)^(1/0.4) // ^ 1/8
abs(x) = 3^0.3125*3.5^(-5/8)*v^0.3125
x = 3^0.3125*3.5^(-5/8)*v^0.3125 or x = -(3^0.3125*3.5^(-5/8)*v^0.3125)
x in { 3^0.3125*3.5^(-5/8)*v^0.3125, -(3^0.3125*3.5^(-5/8)*v^0.3125) }
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