WEBVTT

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Here is the problem: differentiate x cubed cos(2x).

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Find the derivative of x cubed cos(2x) with respect to x.

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Product rule: (uv) prime equals u prime v plus uv prime.

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Chain rule — d by d x[cos u] equals negative sin u times u prime.

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Constant multiple rule.

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Power rule: d by d x[x to the power n] equals n times x to the power n minus 1.

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Combine and simplify.

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So the answer is f prime of x equals 3x squared times cos(2x) minus 2x cubed times sin(2x).

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The derivative is built from the outside in: sums split into terms, constants factor out, products and quotients use their rules, and every composite function multiplies by the derivative of its inside (chain rule).

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Cross-checked numerically against an independent symbolic differentiator at 8 sample points, which checks out.