lean_workbook_plus_19445

⚠️ 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.

John borrows \1,000,000$from the bank. If a bank charges$ 10%$interest (compounded monthly). What will John pay back after$ 20$ years.

Why this node was retired

The posted statement is

theorem lean_workbook_plus_19445 (P : ℕ → ℝ) (hP : P 0 = 1000000) (hP1 : ∀ n, P (n + 1) = (1 + 0.1 / 12) * P n) : P 20 = 7328073.63   :=  by sorry

The recurrence steps are months but the conclusion uses20 steps for20 years; a rounded monetary amount is asserted exactly.

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

Proposed corrected statement

For monthly compounding at nominal annual10%, evaluate P_240=1000000·(121/120)^240. If giving currency to cents, state an explicit rounding rule rather than exact equality to the displayed decimal.

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.