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D( y )
-y = 0
-y = 0
y = 0
y in (-oo:0) U (0:+oo)
13/(-y) = 2 // - 2
13/(-y)-2 = 0
-13*y^-1 = 2 // : -13
y^-1 = -2/13
-1 < 0
1/(y^1) = -2/13 // * y^1
1 = -2/13*y^1 // : -2/13
-13/2 = y^1
y = -13/2
y = -13/2
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