11x13 vs Baselproblem

A short problem, nine answers, one correct — 11x13 trains mental math with no clutter and no words in the problems. THE BASIC OPERATIONS ARE FREE. NOW AND FOREVER. Addition, subtraction, multiplication and division with level bands that grow with you: from small times tables over carrying to the 11–19 tables — the app's namesake — plus chained problems like 47 + 8 + 6 at high levels. WITH “EVERYTHING” YOU PRACTICE EVERY OPERATION. Unlock every package permanently with a one-time purchase or for the duration of a subscription. The 14-day app trial ends automatically without starting a subscription: • Percentages: from 50% of round numbers to surcharge and discount • Roots: squares and cube roots • Powers: up to powers of two and ten • Derivatives and integrals: from monomials to binomials • Geometry: symbol equations (▲ + ▲ = 12), counting corners, matching shapes • Fractions: adding like fractions, simplifying, unlike denominators, multiplying Your progress grows per operation, syncs via iCloud, with progress charts and a Game Center leaderboard. Easy to keep at it: a daily practice reminder at the time you choose, a Home Screen widget with the problem of the day, and statistics with streak, week-over-week comparison and band history. With a true iPad layout and hardware keyboard support (keys 1–9). Subscriptions renew automatically until cancelled in App Store subscriptions. A one-time purchase provides permanent access. Mistake notebook: The last 100 tasks you answered incorrectly are saved on this device. Review up to ten tasks without time pressure. A task leaves the notebook after a correct first answer in two separate rounds. EULA: https://www.apple.com/legal/internet-services/itunes/dev/stdeula/

The Basel problem is a problem in mathematical analysis with relevance to number theory, first posed by Pietro Mengoli in 1650 and solved by Leonhard Euler in 1734.