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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
21/y-8 = 3/1 // - 3/1
21/y-(3/1)-8 = 0
21/y-8-3 = 0
21*y^-1 = 11 // : 21
y^-1 = 11/21
-1 < 0
1/(y^1) = 11/21 // * y^1
1 = 11/21*y^1 // : 11/21
21/11 = y^1
y = 21/11
y = 21/11
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