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D( v )
v = 0
v = 0
v = 0
v in (-oo:0) U (0:+oo)
8/v = 4/5 // - 4/5
8/v-(4/5) = 0
8/v-4/5 = 0
8*v^-1 = 4/5 // : 8
v^-1 = 1/10
-1 < 0
1/(v^1) = 1/10 // * v^1
1 = 1/10*v^1 // : 1/10
10 = v^1
v = 10
v = 10
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