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r in (-oo:+oo)
36/21 = r/28 // - r/28
36/21-(r/28) = 0
12/7-1/28*r = 0 // - 12/7
-1/28*r = -12/7 // : -1/28
r = -12/7/(-1/28)
r = 48
r = 48
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