________ Gian ________ Tara are studying Chinese history now. They find it rich and amazing.
A:Both; and
B:Not only; but also
C:Either; or
D:Neither; nor
①f(1)+2f(1)+…+nf(1);
② fn+(n+1)2 ;
③n(n+1);
④n(n+1)f(1).
其中与f(1)+f(2)+…+f(n)(n∈N*)相等的是____.
过双曲线 x2a2−y2b2=1(a>0,b>0) 的左焦点F作直线l与双曲线交于A,B两点,使得|AB|=4b,若这样的直线有且仅有两条,则离心率e的取值范围是( )