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f in (-oo:+oo)
(-4/5)*f+1 = -13 // + 13
(-4/5)*f+1+13 = 0
14-4/5*f = 0 // - 14
-4/5*f = -14 // : -4/5
f = -14/(-4/5)
f = 35/2
f = 35/2
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