等差数列{an}的前n项和为Sn,已知S10=0,S15=25,则nSn的最小值为________.
若数列{an}的前n项和为Sn=an+,则数列{an}的通项公式是an=________.
设Sn为等差数列{an}的前n项和,S8=4a3,a7=-2,则
a9= ( ).
A.-6 B.-4
C.-2 D.2
等比数列x,3x+3,6x+6,…的第四项等于( ).
A.-24 B.0
C.12 D.24
4cos 50°-tan 40°=________.
如图,在△ABC中,已知点D在BC边上,AD⊥AC,sin∠BAC=,AB=3,AD=3,则BD的长为______.