Question

平行向量a与b的夹角60度,a= ( 2,0 )| b |=1则|a+2b | A.根号3B.2倍根号3C.4D.12选

Answer 1

let b(x,y)

|a+2b|^2 = |a|^2 + 4|b|^2 +4a.b

= 4+4+4(2)(1)cos60度

= 12

|a+2b|= 2√3