5. [9 Pts] Let $P(n)$ be the statement that $n$-cent postage can be formed using just 4-cent and 7-cent stamps. Prove that $P(n)$ is true for all $n ge 18$, using the steps below. (a) First, prove $P(n)$ by regular induction. State your basis step and inductive step clearly and prove them. (b) Now, prove $P(n)$ by strong induction. Again, state and prove your basis step and inductive step. Your basis step should have multiple cases.
Added by Christopher V.
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Step 1
P(18): 18 = 4(4) + 7(1) P(19): 19 = 4(3) + 7(1) P(20): 20 = 4(5) P(21): 21 = 7(3) So, the basis step holds. Inductive Step: Assume P(n) is true for some n ≥ 18. We need to show that P(n+1) is also true. Since P(n) is true, we can write n = 4a + 7b for some Show more…
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