13389844323
幼苗
共回答了14个问题采纳率:92.9% 举报
A • B = |A||B|cosθ,这公式貌似不管用,前提是要知道θ的值,应用纯量积在坐标上的运算
设A = xi + yj,B = mi + nj
则A • B = (xi + yj) • (mi + nj)
= (xi + yj) • mi + (xi + yj) • nj
= (xi • mi + yj • mi) + (xi • nj + yj • nj)
= (xm + 0) + (0 + yn)
= xm + yn
1年前
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13389844323
i • i = j • j = |i|² = |j|² = 1,同方向 i • j = j • i = 0,i和j互相垂直 xi • mi = xm(i • i) = xm yj • mi = ym(j • i) = ym(0) = 0 要用那个公式,要么先知道θ的值,求得A • B;要么先知道A • B的值,求得cosθ A • B = |A||B|cosθ <==> cosθ = (A • B)/(|A||B|),0 ≤ θ ≤ 180° 若A = x₁i + y₁j,B = x₂i + y₂j 则A • B = x₁x₂ + y₁y₂ cosθ = (x₁x₂ + y₁y₂)/[√(x₁² + y₁²)√(x₂² + y₂²)]