Home›Calculus 2›Tangent lines to polar curves›Problem 8.466

Tangent lines to polar curves

Problem 8.466 · medium

Find the slope of the tangent line to \( \displaystyle r = 3 \) at \( \displaystyle \theta = \frac{\pi}{4} \).

With x = r cos θ and y = r sin θ, dy/dx = (dy/dθ)/(dx/dθ).
Reviewed
\[ \frac{d}{d \theta} 3 \cos{\left(\theta \right)} = - 3 \sin{\left(\theta \right)} \]
dx/dθ.✓ Proved
\[ \frac{d}{d \theta} 3 \sin{\left(\theta \right)} = 3 \cos{\left(\theta \right)} \]
dy/dθ.✓ Proved
\[ \frac{\left. 3 \cos{\left(\theta \right)} \right|_{\substack{ \theta=\frac{\pi}{4} }}}{\left. - 3 \sin{\left(\theta \right)} \right|_{\substack{ \theta=\frac{\pi}{4} }}} = -1 \]
At θ = pi/4.✓ Proved

Answer \( -1 \)

Lines: 3 proved, 1 reviewed. 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	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	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
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	the secant through two points on the curve 2·10⁻¹⁰ apart

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly applies the formula for the slope of a tangent line in polar coordinates and performs the substitution accurately.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-10-19 — The solution correctly applies the formula for the slope of a tangent line in polar coordinates and performs the substitution accurately.
gpt-oss:20b: pass 2026-10-19
qwen3.6:27b-mlx: pass 2026-10-16 — The solution correctly applies the formula for the slope of a tangent line in polar coordinates and performs the substitution accurately.
gpt-oss:20b: pass 2026-10-16
qwen3.6:27b-mlx: fail (error) 2026-10-15 — The solution calculates dx/dy instead of dy/dx. The formula dy/dx = (dy/dθ)/(dx/dθ) requires dividing the derivative of y by the derivative of x, but line 4 divides the expression for dx/dθ by the expression for dy/dθ.
gpt-oss:20b: pass 2026-10-15

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/polar_tangent, checked 2026-10-19 with SymPy 1.14.0.