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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
3 = 12/r // - 12/r
3-(12/r) = 0
3-12*r^-1 = 0
-12*r^-1 = -3 // : -12
r^-1 = 1/4
-1 < 0
1/(r^1) = 1/4 // * r^1
1 = 1/4*r^1 // : 1/4
4 = r^1
r = 4
r = 4
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