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r in (-oo:+oo)
(r/3)*r-1 = 4 // - 4
(r/3)*r-4-1 = 0
1/3*r^2 = 5 // : 1/3
r^2 = 15
r^2 = 15 // ^ 1/2
abs(r) = 15^(1/2)
r = 15^(1/2) or r = -15^(1/2)
r in { 15^(1/2), -15^(1/2) }
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