If you gain five pounds a year, every year, your "rate of growth" will steadily shrink, too.

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The two-watt Atom chip inside the Series 5 is rated for about nine hours of battery life, whereas the Core i5-powered Series 7 is expected to last between four and five hours on a charge. (The Series 5 is lighter, too, at 1.65 pounds, compared with 1.89 for the Series 7.) And, being a lower-powered device, the Series 5 has no vents.

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