Worked example: a derivative
Written by hand in the Detailed teaching layout, shortened to the main lines:
- Problem: Find f'(x) for f(x) = x^2 sin(x)
- Idea: f is a product of two functions, so use the product rule (uv)' = u'v + uv'
- 1. u = x^2, so u' = 2x
- 2. v = sin(x), so v' = cos(x)
- 3. f'(x) = 2x sin(x) + x^2 cos(x)
- Check: at x = π, f'(π) = 0 + π^2 × (-1) = -π^2 ≈ -9.87, which matches the slope of f measured numerically near π
- Answer: f'(x) = 2x sin(x) + x^2 cos(x)
- Common mistake: multiplying the two derivatives, 2x cos(x), which ignores the product rule




