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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
1/15 = 1/12-(1/y) // - 1/12-(1/y)
1/y+1/15-(1/12) = 0
1/y+1/15-1/12 = 0
1*y^-1 = 1/60 // : 1
y^-1 = 1/60
-1 < 0
1/(y^1) = 1/60 // * y^1
1 = 1/60*y^1 // : 1/60
60 = y^1
y = 60
y = 60
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