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r in (-oo:+oo)
r/13 = r-(3/10) // - r-(3/10)
r/13-r+3/10 = 0
3/10-12/13*r = 0 // - 3/10
-12/13*r = -3/10 // : -12/13
r = -3/10/(-12/13)
r = 13/40
r = 13/40
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