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Tangent lines

Problem 2.1268 · medium Video

Find an equation of the tangent line to \( \displaystyle y = - x - 3 \sin{\left(x \right)} + 1 \) at \( \displaystyle x = \pi \).

The tangent line passes through the point (a, f(a)) and has slope f'(a).
\[ \left. - x - 3 \sin{\left(x \right)} + 1 \right|_{\substack{ x=\pi }} = 1 - \pi \]
The point of tangency.✓ Proved
\[ \frac{d}{d x} \left(- x - 3 \sin{\left(x \right)} + 1\right) = - 3 \cos{\left(x \right)} - 1 \]
Differentiate.✓ Proved
\[ \left. - 3 \cos{\left(x \right)} - 1 \right|_{\substack{ x=\pi }} = 2 \]
The slope at the point.✓ Proved
\[ 2 x - 3 \pi + 1 \]
Point-slope form, then simplify.

Answer \( y = 2 x - 3 \pi + 1 \)

The graph of y = -x - 3sin(x) + 1 near x = pi. A secant line through the point (pi, 1 - pi) and a second point on the curve turns into the tangent line as the second point slides in; its slope settles at 2, giving y = 2x - 3pi + 1.

Lines: 3 proved, 2 not checked. 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. How we verify

The full receipt

Line	Status	Checked by	Detail
1	Not checked	—	a sentence; read, not computed
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
5	Not checked	—	both sides are the same expression: the line states a value, and the working that produced it is not on the line
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	the line meets the curve at x = a, and its slope matches a central difference quotient of f there

Reviewers

gpt-oss:20b: fail (error) — The point of tangency is (π, 1 − π), but the solution writes the point‑slope form incorrectly. The correct tangent line is y = 2x − 3π + 1, not the expression derived in step 5.
qwen3.6:27b-mlx: pass — The solution correctly identifies the point of tangency and the slope, and the final equation is algebraically equivalent to the stated answer.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly identifies the point of tangency and the slope, and the final equation is algebraically equivalent to the stated answer.
gpt-oss:20b: fail (error) 2026-10-03 — The point of tangency is (π, 1 − π), but the solution writes the point‑slope form incorrectly. The correct tangent line is y = 2x − 3π + 1, not the expression derived in step 5.
qwen3.6:27b-mlx: pass 2026-09-29 — The solution correctly identifies the point of tangency and the slope, and applies the point-slope formula accurately to derive the final equation.
gpt-oss:20b: pass 2026-09-29
qwen3.6:27b-mlx: inconclusive 2026-09-29 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "The derivative in step 3 is incorrect; the derivative of -x is -1, so f'(x) should be -1 - 3*cos(x), not -3*cos(x) - 1 (which implies the derivative
gpt-oss:20b: pass 2026-09-29

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/tangent_line, checked 2026-10-13 with SymPy 1.14.0.