A:broke off
B:broke i
C:broke out
D:broke dow
如右图,在△ABC中,∠C=90°,AB的垂直平分线MN分别交AC,AB于点D,E.若∠CBD : ∠DBA =3:1,则∠A为( ).
cos(﹣510°)的值为( )
下列关于宋代城市的说法有误的是( )
先化简,再求值:(xx+y﹣x2x2+2xy+y2)÷(xx+y﹣x2x2-y2),其中x=2,y=1.