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D( r )
r+55 = 0
r+55 = 0
r+55 = 0
r+55 = 0 // - 55
r = -55
r in (-oo:-55) U (-55:+oo)
21 = (55*r)/(r+55)+12 // - (55*r)/(r+55)+12
21-((55*r)/(r+55))-12 = 0
21-55*r*(r+55)^-1-12 = 0
(-55*r)/(r+55)-12+21 = 0
(-55*r)/(r+55)+(-12*(r+55))/(r+55)+(21*(r+55))/(r+55) = 0
21*(r+55)-12*(r+55)-55*r = 0
21*r-67*r-660+1155 = 0
495-46*r = 0
(495-46*r)/(r+55) = 0
(495-46*r)/(r+55) = 0 // * r+55
495-46*r = 0
495-46*r = 0 // - 495
-46*r = -495 // : -46
r = -495/(-46)
r = 495/46
r = 495/46
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