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