题目

已知|a|＝2|b|，且|b|≠0，关于x的方程x2＋|a|x－a·b＝0有两相等实根，则向量a与b的夹角是( ) (A)－  (B)－  (C)  (D) 答案：D.设向量a与b的夹角为θ，由方程x2＋|a|x－a·b＝0有两相等的实根可得Δ＝|a|2＋4a·b＝0，即4|b|2＋8|b|2cosθ＝0，∴cosθ＝－， 则向量a与b的夹角为.(2017，兰州) When you ________ me last night, I ________ the piano.A. were calling；was playing B. were calling；played C. called；was playing