Average rate of change and secant lines
Problem 2.1624 · easy
Find the average rate of change of \( \displaystyle f(x) = 3 x^{2} + x \) on \( \displaystyle [1, 5] \), and the equation of the secant line through the endpoints.
- \[ \left. 3 x^{2} + x \right|_{\substack{ x=1 }} = 4 \]The value at the left endpoint.✓ Proved
- \[ \left. 3 x^{2} + x \right|_{\substack{ x=5 }} = 80 \]The value at the right endpoint.✓ Proved
- \[ 19 \]Change in output over change in input.✓ Proved
- \[ 19 x - 15 \]Point-slope form through the left endpoint.✓ Proved
Answer \( \frac{\Delta f}{\Delta x} = 19,\quad y = 19 x - 15 \)
Every line of this solution was proved by the computer algebra system SymPy. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | recomputed in floating point |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: fail (error) — The solution fails to explicitly calculate the average rate of change (slope) as (80-4)/(5-1), jumping directly to the result 19 without showing the division. It also fails to show the derivation of the secant line equation from the point-slope formula, merely stating the final equation.
Every verdict on record (4)
qwen3.6:27b-mlx: fail (error) 2026-10-05 — The solution fails to explicitly calculate the average rate of change (slope) as (80-4)/(5-1), jumping directly to the result 19 without showing the division. It also fails to show the derivation of the secant line equation from the point-slope formula, merely stating the final equation.gpt-oss:20b: pass 2026-10-05qwen3.6:27b-mlx: fail (error) 2026-10-05 — The solution lacks the explicit calculation of the average rate of change (slope) and the derivation of the secant line equation, relying instead on tautological equations that do not demonstrate the work.gpt-oss:20b: pass 2026-10-05
Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate
method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence,
not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by
generator:structured/secant_slope, checked 2026-10-05 with SymPy 1.14.0.