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D( w )
w = 0
w = 0
w = 0
w in (-oo:0) U (0:+oo)
90/w = 6 // - 6
90/w-6 = 0
90*w^-1 = 6 // : 90
w^-1 = 1/15
-1 < 0
1/(w^1) = 1/15 // * w^1
1 = 1/15*w^1 // : 1/15
15 = w^1
w = 15
w = 15
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