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Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works
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Second Derivative Test Mastery

Classify a critical point by the sign of $f''$ โ€” four in a row.

๐ŸŽฏ 4 in a row ๐Ÿšซ no clock
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