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f in (-oo:+oo)
(-13/20)*f = 8/20 // - 8/20
(-13/20)*f-(8/20) = 0
(-13/20)*f-2/5 = 0
-13/20*f-2/5 = 0 // + 2/5
-13/20*f = 2/5 // : -13/20
f = 2/5/(-13/20)
f = -8/13
f = -8/13
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