3.如图,将矩形纸片ABCD按如图所示的方式折叠,得到菱形AECF,若AB=6,则BC的长为( )

| A. | 2 | B. | 2$\sqrt{2}$ | C. | 4 | D. | 2$\sqrt{3}$ |
20.观察下列式子:①$\sqrt{2-\frac{2}{5}}=2\sqrt{\frac{2}{5}}$;②$\sqrt{3-\frac{3}{10}}=3\sqrt{\frac{3}{10}}$;③$\sqrt{4-\frac{4}{17}}=4\sqrt{\frac{4}{17}}$;④$\sqrt{5-\frac{5}{26}}=5\sqrt{\frac{5}{26}}$;…请你按照规律写出第n(n≥1)个式子是( )
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| A. | $\sqrt{n-1-\frac{n-1}{(n-1)^{2}+1}}$=(n-1)$\sqrt{\frac{n-1}{(n-1)^{2}+1}}$ | B. | $\sqrt{n-\frac{n}{{n}^{2}-1}}=n\sqrt{\frac{n}{{n}^{2}-1}}$ | ||
| C. | $\sqrt{n+1-\frac{n+1}{(n+1)^{2}+1}}$=(n+1)$\sqrt{\frac{n+1}{(n+1)^{2}+1}}$ | D. | $\sqrt{n-\frac{n}{{n}^{2}+1}}=n\sqrt{\frac{n}{{n}^{2}+1}}$ |