ta có: \(\frac{3^{40}-1}{3^{40}+1}=\frac{3^{40}+1-2}{3^{40}+1}=1-\frac{2}{3^{40}+1}.\)
\(\frac{3^{50}-1}{3^{50}+1}=1-\frac{2}{3^{50}+1}\)
\(\Rightarrow\frac{2}{3^{40}+1}>\frac{2}{3^{50}+1}\)
\(\Rightarrow1-\frac{2}{3^{40}+1}< 1-\frac{2}{3^{50}+1}\)
\(\Rightarrow\frac{3^{40}-1}{3^{40}+1}< \frac{3^{50}-1}{3^{50}+1}\)