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D( y )
y = 0
y-6 = 0
y = 0
y = 0
y-6 = 0
y-6 = 0
y-6 = 0 // + 6
y = 6
y in (-oo:0) U (0:6) U (6:+oo)
90/y-(90/(y-6)) = 2/y // - 2/y
90/y-(90/(y-6))-(2/y) = 0
90/y-90*(y-6)^-1-2*y^-1 = 0
90/y-90/(y-6)-2/y = 0
(-90*y)/(y*(y-6))+(90*(y-6))/(y*(y-6))+(-2*(y-6))/(y*(y-6)) = 0
90*(y-6)-2*(y-6)-90*y = 0
12-2*y-540 = 0
-2*y-528 = 0
(-2*y-528)/(y*(y-6)) = 0
(-2*y-528)/(y*(y-6)) = 0 // * y*(y-6)
-2*y-528 = 0
-2*y-528 = 0 // + 528
-2*y = 528 // : -2
y = 528/(-2)
y = -264
y = -264
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