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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
12/60 = -15/y // + -15/y
12/60-(-15/y) = 0
15*y^-1+12/60 = 0
15*y^-1 = -1/5 // : 15
y^-1 = -1/75
-1 < 0
1/(y^1) = -1/75 // * y^1
1 = -1/75*y^1 // : -1/75
-75 = y^1
y = -75
y = -75
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