Some also noted that Facebook advertising is still relatively experimental and that CMOs can probably justify any decision about it on the grounds of some sort of return-on-investment calculus, at least from a short-term perspective.

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The details of their journey led me to a three-year obsession with the calculus of achievement.

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He takes a full boat of courses, a few AP classes and pre-calculus, English and Spanish II.

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And a similar--but less extreme--calculus applies to other companies being touted as potential Biogen buyers.

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And remember, this is - really comes down to a very simple calculus.

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A-level: chiefly geometry, vectors and calculus.

ECONOMIST: Exams

A-level too: it's possible to get an A in maths without a solid grasp of calculus, for example.

ECONOMIST: Exams

They regularly drop in on Mr Gold, who quizzes them closely, his wild greying locks and well-stuffed shirt pocket giving him the air of a rumpled physics professor expounding an exotic retail calculus.

ECONOMIST: Face value

Driving the calculus are China, Russia and other emerging markets, where a small but fast-growing millionaire class is flocking to big, high priced and lavishly loaded models that can handle poor roads as well as a family.

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Khan, 36, started Khan Academy in a closet at his home in Silicon Valley, but with backing from the likes of Bill Gates and venture capitalist John Doerr and his wife Ann, has expanded into a 36-person company serving 10 million students around the world with free video talks on the Internet explaining everything from calculus to chemistry.

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