A:By boat.
B:By train
C:By plane.
①求数列{an}的通项公式;
②设bn=anlog2an,求数列{bn}的前n项和Sn.
如图,四边形ABCD内接于⊙O,AB、DC延长线交于N,AD、BC的延长线交于M,∠M=40°,∠N=20°,则∠A是( )