题目

已知||=|a|=3,||=|b|=3,∠AOB=60°,求|a+b|. 答案：解:如图,∵||=||=3,∴四边形OACB为菱形.连OC、AB,则OC⊥AB,∵∠AOB=60°,∴AB=||=3.∴在Rt△BDC中,CD=.∴||=|a+b|=×2=.________ was 1998 ________ I graduated from the university. [     ]A. It; when  B. It; that  C. That; when  D. That; that