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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
3.8/y = 6/12.1 // - 6/12.1
3.8/y-(6/12.1) = 0
3.8/y-0.49586777 = 0
3.8*y^-1 = 0.49586777 // : 3.8
y^-1 = 0.13049152
-1 < 0
1/(y^1) = 0.13049152 // * y^1
1 = 0.13049152*y^1 // : 0.13049152
7.66333322 = y^1
y = 7.66333322
y = 7.66333322
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