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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
12 = 3600/r // - 3600/r
12-(3600/r) = 0
12-3600*r^-1 = 0
-3600*r^-1 = -12 // : -3600
r^-1 = 1/300
-1 < 0
1/(r^1) = 1/300 // * r^1
1 = 1/300*r^1 // : 1/300
300 = r^1
r = 300
r = 300
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