cosα = 1/√(1+2+1) = 1/2
cosβ = √2/√(1+2+1) = √2/2
cosγ = 1/√(1+2+1) = 1/2
∂u/∂x = y² - yz = 3
∂u/∂y = 2xy - xz = 0
∂u/∂z = 3z² - xy = 12
方向导数
=1/2 * 3 + √2/2 * 0 + 1/2 * 12
=15/2