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b in (-oo:+oo)
(3/5)*b+(3/7)*b+1/2 = 89/70 // - 89/70
(3/5)*b+(3/7)*b+1/2-(89/70) = 0
(3/5)*b+(3/7)*b+1/2-89/70 = 0
36/35*b-27/35 = 0 // + 27/35
36/35*b = 27/35 // : 36/35
b = 27/35/36/35
b = 3/4
b = 3/4
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