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D( v )
v = 0
v = 0
v = 0
v in (-oo:0) U (0:+oo)
9/v = 4 // - 4
9/v-4 = 0
9*v^-1 = 4 // : 9
v^-1 = 4/9
-1 < 0
1/(v^1) = 4/9 // * v^1
1 = 4/9*v^1 // : 4/9
9/4 = v^1
v = 9/4
v = 9/4
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