Let Pn be the statement : any postage of n >= 2 cents can be made of(adsbygoogle = window.adsbygoogle || []).push({});

3-cent and 2-cent stamps. What is wrong with the following proof of

Pn by induction? How can it be fixed without changing the induction

step much?

Base case : 2 = 2 and so P2 is true.

Induction step : Fix some n >=2. Assume that Pk is true for k <= n, we

will prove Pn+1.

Since Pn-1 is true, we know that

n - 1 = a * 2 + b * 3

and hence

n + 1 = (a+1)*2+b*3

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# Help proof by induction

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