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f in (-oo:+oo)
90 = (5/9)*(f-32) // - (5/9)*(f-32)
90-((5/9)*(f-32)) = 0
(-5/9)*(f-32)+90 = 0
90-5/9*(f-32) = 0
90-5/9*(f-32) = 0
970/9-5/9*f = 0
970/9-5/9*f = 0
970/9-5/9*f = 0 // - 970/9
-5/9*f = -970/9 // : -5/9
f = -970/9/(-5/9)
f = 194
f = 194
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