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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
15/y-(3/y) = 4 // - 4
15/y-(3/y)-4 = 0
15/y-3*y^-1-4 = 0
12*y^-1 = 4 // : 12
y^-1 = 1/3
-1 < 0
1/(y^1) = 1/3 // * y^1
1 = 1/3*y^1 // : 1/3
3 = y^1
y = 3
y = 3
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