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D( v )
v = 0
v = 0
v = 0
v in (-oo:0) U (0:+oo)
5/140 = 12/v // - 12/v
5/140-(12/v) = 0
5/140-12*v^-1 = 0
-12*v^-1 = -1/28 // : -12
v^-1 = 1/336
-1 < 0
1/(v^1) = 1/336 // * v^1
1 = 1/336*v^1 // : 1/336
336 = v^1
v = 336
v = 336
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