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d in (-oo:+oo)
(4/5)*d = 2/5 // - 2/5
(4/5)*d-(2/5) = 0
(4/5)*d-2/5 = 0
4/5*d-2/5 = 0 // + 2/5
4/5*d = 2/5 // : 4/5
d = 2/5/4/5
d = 1/2
d = 1/2
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