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x in (-oo:+oo)
(1/8)*x+(2/5)*x = 1 // - 1
(1/8)*x+(2/5)*x-1 = 0
21/40*x-1 = 0 // + 1
21/40*x = 1 // : 21/40
x = 1/21/40
x = 40/21
x = 40/21
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