|a|*|b|*cos θ = 1/2
|b|*cos θ = 1/2
(a-b) (a+b) = |a|^2 - |b|^2 = 1/2
1-|b|^2 = 1/2
|b|=1/开方2
代入可得 1/开方2*cos θ = 1/2
θ = 45°
上面用*来代替 点