x +100=500+400

300 : 3 + x = 200

   100   + x = 200

              x = 200 -100

              x = 100

Ta có:

\(\frac{1}{a}+\frac{1}{b}=\frac{1}{a+b}\)

\(\Leftrightarrow\frac{a+b}{ab}=\frac{1}{a+b}\Leftrightarrow\frac{\left(a+b\right)^2}{ab}=1\)

\(\Leftrightarrow\frac{a^2+2ab+b^2}{ab}=1\Leftrightarrow\frac{a^2+b^2}{ab}+2=1\)

\(\Leftrightarrow\frac{a}{b}+\frac{b}{a}=1-2=-1\)

Vậy \(\frac{a}{b}+\frac{b}{a}=-1\)

18 dm2 nha

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sao là j