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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
50/y = 40/x // - 40/x
50/y-(40/x) = 0
50/y-40*x^-1 = 0
-40*x^-1 = -(50*y^-1) // : -40
x^-1 = (5*y^-1)/4
-1 < 0
1/(x^1) = (5*y^-1)/4 // * x^1
1 = x^1*((5*y^-1)/4) // : (5*y^-1)/4
(4*y)/5 = x^1
x = (4*y)/5
x = (4*y)/5
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