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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
6/14 = 9/y // - 9/y
6/14-(9/y) = 0
6/14-9*y^-1 = 0
-9*y^-1 = -3/7 // : -9
y^-1 = 1/21
-1 < 0
1/(y^1) = 1/21 // * y^1
1 = 1/21*y^1 // : 1/21
21 = y^1
y = 21
y = 21
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