Who can analyze the trihedral angle formula? Thank you for your help

Who can analyze the trihedral angle formula? Thank you for your help

As shown in the figure, OA is the oblique line of plane α, AB is the projection of OA in the plane, AC is any straight line passing through point a in plane α, let ∠ OAB = θ 1, ∠ BAC = θ 2, ∠ OAC = θ, then cos θ = cos θ 1 * cos θ 2

Multivariate function without stationary point, there will be extremum? Which high number shrimp trouble answer, is multivariate Oh, not a single ~ 3Q~

The extremum of multivariate function may not be stationary point
Because, at this point, it is possible that the partial derivatives in all directions do not exist

The stationary point of multivariate function is

Z = 2x ∧ 2 + 3Y ∧ 2 is a quadratic equation with two variables. If the derivatives of X and y are equal to 0 at the same time, the obtained (x, y), that is, (0,0), is a stationary point, but not necessarily a pole. To verify, we need the second derivative, a = Z "(XX) = 4, B = Z" (XY) = 0, C = Z "(YY) = 6, B ^ 2-ac = - 240, then the modified point is not a pole