Which innovative product strategy did Stewart Calculus use to become the dominant calculus textbook in North American universities, revolutionizing mathematics textbook marketing?

While mathematics textbooks might seem like purely academic products, they represent a multi-billion dollar industry with fascinating product development and marketing strategies. Publishers compete fiercely to capture market share in educational institutions worldwide. This trivia question focuses on a pivotal product innovation that revolutionized how calculus textbooks were marketed and sold to universities in the late 1990s and early 2000s.

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Creating the first online homework system (WebAssign) bundled with textbooks
Offering unprecedented revenue-sharing with mathematics departments that adopted the textbook
Pioneering the first full-color calculus textbook with digital animations
Developing a modular approach allowing professors to customize chapter sequences