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D( y )
y = 0
y = 0
y = 0
y in (-oo:0) U (0:+oo)
42/12 = 21/y // - 21/y
42/12-(21/y) = 0
42/12-21*y^-1 = 0
-21*y^-1 = -7/2 // : -21
y^-1 = 1/6
-1 < 0
1/(y^1) = 1/6 // * y^1
1 = 1/6*y^1 // : 1/6
6 = y^1
y = 6
y = 6
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