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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
18.434 = 20.1/y // - 20.1/y
18.434-(20.1/y) = 0
18.434-20.1*y^-1 = 0
-20.1*y^-1 = -18.434 // : -20.1
y^-1 = 0.91711443
-1 < 0
1/(y^1) = 0.91711443 // * y^1
1 = 0.91711443*y^1 // : 0.91711443
1.09037648 = y^1
y = 1.09037648
y = 1.09037648
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