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Unit 3 · 8.3.1–8.3.3 · First derivatives

Gradient & product rule

The gradient collects all partial derivatives in one vector. It obeys the product rule ∇(fg) = (∇f)·g + (∇g)·f — scalar times vector, not a scalar product.

Unit 3 · 8.3.1–8.3.3
Formulas at a glance · First derivatives

Partial derivatives

∂f∂xi(x)=limh→0f(x+hei)−f(x)h

Gradient & product rule

∇f(x)=(∂f∂x1(x),…,∂f∂xd(x))

Total derivative

limy→0f(x+y)−f(x)−dfx(y)‖y‖=0

Tangent plane & linearization

z=f(x0,y0)+fx(x0,y0)(x−x0)+fy(x0,y0)(y−y0)

Directional derivatives & steepest ascent

Dvf(x)=⟨∇f(x),v⟩,|Dvf(x)|≤‖∇f(x)‖

Quadratic forms xᵀCx

∇f(x)=x⊤(C+C⊤)=((C+C⊤)x)⊤