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r in (-oo:+oo)
-6 = r-((4/5)*(3-r)) // - r-((4/5)*(3-r))
(4/5)*(3-r)-r-6 = 0
4/5*(3-r)-r-6 = 0
4/5*(3-r)-r-6 = 0
12/5-9/5*r-6 = 0
-9/5*r-18/5 = 0
-9/5*r-18/5 = 0
-9/5*r-18/5 = 0 // + 18/5
-9/5*r = 18/5 // : -9/5
r = 18/5/(-9/5)
r = -2
r = -2
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