[tex]\text{Folosim formula:}\ \log_{a}b+\log_{a} c=\log _{a} {b\cdot c}\\
\text{Deci vom avea:}\\
\log_{2}\dfrac{2}{3}+\log_{2}\dfrac{3}{4}+\dots+\log_{2} \dfrac{7}{8}=\log_{2} \left(\dfrac{2}{3}\cdot \dfrac{3}{4}\cdot _{\dots} \cdot \dfrac{7}{8}\right)=\log_{2} \dfrac{2}{8}=\boxed{-2}[/tex]