题目

 如图（1），∠ABC=90°，O为射线BC上一点，OB=4，以点O为圆心，BO长为半径作⊙O交BC于点D、E.（1）当射线BA绕点B按顺时针方向旋转多少度时与⊙O相切？请说明理由.（2）若射线BA绕点B按顺时针方向旋转与⊙O相交于M、N两点（如图（2）），MN=2，求弧MN的长. 答案： （1）60°，理由如下：设AB与⊙O相切于点F 连结OF，则OF⊥AB， 又∵BO=4      ∴ ∴射线BA绕B点顺时针旋转 （2）过点O作， 则 又∵      ∴ ∴    ∴ ∴根据短文内容判断正误。U2 is the name of a band from Ireland. It's one of the most famous rock bands in the world.In l978, a l7-year-old high school student, Larry Mullen, looked for some musicians to make a band. Finally, he found another three boys and they set up a band. The four boys named their band U2.They travelled a lot and gave concerts in many countries. More and more people got to know them and love their music. Many of their songs, such as Beautiful Day and Walk On, were popular. In the l980s, the band became famous.Now the four members of U2 are still good friends and their music is still popular. People in the world still enjoy their music.（1）U2 is a famous rock band in the world.（2）There are three members in the band U2.（3）The band became famous in l978.（4）U2's music is still popular in the world.（5）The article is about a rock band.