Question

已知向量a=(3,根号3),求向量b,使|b|=2|a|,并且a与b的夹角为π/3.

Answer 1

=(x,y)

向a*向b=|a|*|b|*cosπ/3=12

得①3x+√3y=12

②√ x^2 + √ y^2 = 4√ 3

得 x=6,y=-2√ 3

或 x=0,y=4√ 3