i1+i2+i3+…+i2000=______．
i1+i2+i3+…+i2000
=500×（i-1-i+1）
=500×0
=0．
故答案为：0．
Such films _____ shown yesterday ______ not worth seeing again.
A．that was; is
B．as were; are
C．as were; is
D．those were; are
对于单钩钩身的验算截面应该为（）
A.钩身的垂直截面
B.钩身的水平截面
C.钩身的45度截面
D.A和C
