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f in (-oo:+oo)
f/2 = 2.5 // - 2.5
f/2-2.5 = 0
1/2*f-2.5 = 0 // + 2.5
1/2*f = 2.5 // : 1/2
f = 2.5/1/2
f = 5
f = 5
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