lean_workbook_plus_57777

⚠️ Retired — specification defect

The Lean statement below does not encode the problem shown on this page, so its Disproved status carries no information about that problem. Do not import this node or use it as a dependency.

An investment of $5000$, earning interest compounded semi-annually grew to $9000$ in 15 years. What was the nominal rate?

Why this node was retired

The posted statement is

theorem lean_workbook_plus_57777 (P A : ℝ) (n : ℕ) (hP : P = 5000) (hA : A = 9000) (hn : n = 15) : ∃ r, A = P * (1 + r / 2)^(n * 2) ∧ r = 0.039564   :=  by sorry

The source asks for all solutions or a value; the stored theorem supplies an unsupported answer contradicted by an accepted disproof.

The recorded counterexample refutes the statement as encoded. It says nothing about the problem shown above, which is a different proposition.

Proposed corrected statement

Define the nonnegative nominal annual rate exactly as r=2((9/5)^(1/30)−1), with a real thirtieth root. Report any decimal only with a rounding tolerance, not an exact equality.

Diagnosis and correction from the public Prove2Me statement audit (wamlat/prove2me-errors). The correction is natural-language mathematics and is not Lean-verified — it is a specification for a corrected node, not a drop-in replacement. No corrected replacement node exists yet.