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f in (-oo:+oo)
-12 = f/4+8 // - f/4+8
-(f/4)-12-8 = 0
-1/4*f-20 = 0 // + 20
-1/4*f = 20 // : -1/4
f = 20/(-1/4)
f = -80
f = -80
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