将矩形纸片ABCD按如图所示的方式折叠,得到菱形AECF.若AB=3,则BC的长为____.
已知sinα=-45,π<α<3π2,则cosα2的值为( )
As is known to us all, taking exercise is b____ to our health.
已知xy=13,则x-yy的值为 ____