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MathidiaMathematics Academy
Multivariable calculus
Lesson 33 of 75
Standard practice

Gradient and directional derivative

Objective. Use gradients to find steepest change and tangent planes.

  1. 1. Learn
  2. 2. Worked example
  3. 3. Practice
  4. 4. Feedback
  5. 5. Continue

Learn: the key idea

Gradient and directional derivative is treated as university mathematics: definitions and hypotheses come before procedures. Geometry and differentiation of scalar functions of several variables. Begin by identifying the mathematical objects, notation, domain and assumptions. Develop the central result from first principles or an explicitly stated theorem, and connect symbolic work to geometric, numerical, probabilistic or computational meaning where appropriate. Compare at least two representations, explain why each implication is valid, and preserve exact structure until approximation is justified. Conclude by testing edge cases, dimensions, limiting behaviour and counterexamples so the argument is rigorous, interpretable and reusable in unfamiliar problems.

Worked example

Differentiate x^6.

Differentiate x^6
Illustrated card showing Differentiate x^6
  1. 1Restate the problem in precise notation, list the hypotheses, and identify the definition or mathematical structure controlling the solution.
  2. 2Select a theorem, representation or algorithm and verify that every condition required for its use is satisfied.
  3. 3Carry out the derivation in explicit justified stages, tracking domains, signs, dimensions, convergence and exceptional cases.
  4. 4Verify independently by substitution, a second representation, a limiting case, computation or counterexample analysis. The checked conclusion is 6x^5.

Answer: 6x^5

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Practice

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Common mistake. A frequent error in gradient and directional derivative is to manipulate notation before establishing definitions and hypotheses. This can produce a plausible calculation whose implication, domain, convergence or uniqueness claim is false. Name the governing result, test its conditions, preserve equivalence at each step, and challenge the conclusion with a boundary case or counterexample.

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