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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
6 = 90/y // - 90/y
6-(90/y) = 0
6-90*y^-1 = 0
-90*y^-1 = -6 // : -90
y^-1 = 1/15
-1 < 0
1/(y^1) = 1/15 // * y^1
1 = 1/15*y^1 // : 1/15
15 = y^1
y = 15
y = 15
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