已知向量a＝(2,1)，a·b＝10，|a＋b|＝5，则|b|＝( ) A． B_爱下问搜题

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问题选择题

已知向量a＝(2,1)，a·b＝10，|a＋b|＝5，则|b|＝(　　)

A．

B．

C．5

D．25

答案

答案：C

选择题

Somehow this seems to me to be _______. How could a team that had fought for Canada before suddenly become citizens of the USA?

A．out of question

B．out of place

C．out of shape

D．out of control

判断题

投射线汇交一点的投影法为平行投影法。