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D( v )
v = 0
v = 0
v = 0
v in (-oo:0) U (0:+oo)
4/v = 84 // - 84
4/v-84 = 0
4*v^-1 = 84 // : 4
v^-1 = 21
-1 < 0
1/(v^1) = 21 // * v^1
1 = 21*v^1 // : 21
1/21 = v^1
v = 1/21
v = 1/21
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