Formula Battle is a free quiz game for memorizing Math I formulas from the Japanese high-school curriculum. You defeat 'exam monsters' with the power of formulas in four-choice battles, which keeps even reluctant memorizers moving.
Questions are in Japanese (interface in English). The more you answer, the stronger the monsters — and the faster you find the formulas you only half-know.
Answer the same question correctly twice and it graduates into a mastery card; the ranking counts how many cards you have collected. You can drill only the questions you got wrong, and every day you play adds a study stamp. No sign-up is required and your records stay on your own device. (The questions themselves are in Japanese.)