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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
0.12 = 1800/r // - 1800/r
0.12-(1800/r) = 0
0.12-1800*r^-1 = 0
-1800*r^-1 = -0.12 // : -1800
r^-1 = 0.00006667
-1 < 0
1/(r^1) = 0.00006667 // * r^1
1 = 0.00006667*r^1 // : 0.00006667
14999.2500375 = r^1
r = 14999.2500375
r = 14999.2500375
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