I_{A B} = U_{A B} \cdot \frac{8 R + 8 R}{8 R \cdot 8 R} ⟹ R = \frac{U_{A B}}{4 I} = 100 Ω

${I_{AB} = U_{AB} \cdot \frac{8R + 8R}{8R \cdot 8R} \Longrightarrow R =\frac{U_{AB}}{4I}= 100 \Omega}$

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