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