a(x1,y1) b(x2,y2)
a*b=x1y1+x2y2=|a||b|cosα ("||"是模,不是绝对值)
(a-2b)(2a+b)
=2|a|²-2|b|²-3a*b
=2|a|²-2|b|²-3|a||b|cosα
=-10-3*6*cosα=-1
α=2π/3
a(x1,y1) b(x2,y2)
a*b=x1y1+x2y2=|a||b|cosα ("||"是模,不是绝对值)
(a-2b)(2a+b)
=2|a|²-2|b|²-3a*b
=2|a|²-2|b|²-3|a||b|cosα
=-10-3*6*cosα=-1
α=2π/3