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r in (-oo:+oo)
r/2.8 = 2.7/2.1 // - 2.7/2.1
r/2.8-(2.7/2.1) = 0
r/2.8-1.28571429 = 0
0.35714286*r-1.28571429 = 0 // + 1.28571429
0.35714286*r = 1.28571429 // : 0.35714286
r = 1.28571429/0.35714286
r = 3.59999998
r = 3.59999998
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