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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
80/y-(80/y)+6 = 4/y // - 4/y
80/y-(80/y)-(4/y)+6 = 0
80/y-80*y^-1-4*y^-1+6 = 0
-4*y^-1 = -6 // : -4
y^-1 = 3/2
-1 < 0
1/(y^1) = 3/2 // * y^1
1 = 3/2*y^1 // : 3/2
2/3 = y^1
y = 2/3
y = 2/3
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