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x in (-oo:+oo)
1/u+1/v = 1/f // - 1/f
1/u-(1/f)+1/v = 0
1/u-f^-1+1/v = 0
u^-1-f^-1+v^-1 = 0
x naleu017Cy do R
x in (-oo:+oo)
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