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Concrete mathematics : a foundation for computer science

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2.3 MANIPULATION OF SUMS 33<br />

(When x = 1, the sum is of course simply (n + 1 )a.) The right-hand side<br />

Ah yes, this <strong>for</strong>mula can be remembered as the first term included in the sum minus the first term<br />

was drilled into me excluded (the term after the last), divided by 1 minus the term ratio.<br />

in high school.<br />

That was almost too easy. Let’s try the perturbation technique on a<br />

slightly more difficult sum,<br />

S, = x k2k<br />

O

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