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D( v )
v = 0
v = 0
v = 0
v in (-oo:0) U (0:+oo)
90/v-1 = 15 // - 15
90/v-15-1 = 0
90*v^-1 = 16 // : 90
v^-1 = 8/45
-1 < 0
1/(v^1) = 8/45 // * v^1
1 = 8/45*v^1 // : 8/45
45/8 = v^1
v = 45/8
v = 45/8
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