Here are the proofs using the Principle of Mathematical Induction.
1. Prove that 1+2+3+⋯+n=∑k=1nk=2n(n+1), ∀n≥1.
Let P(n) be the statement 1+2+3+⋯+n=2n(n+1).
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Base Case (n=1):
LHS =1
RHS =21(1+1)=21(2)=1
Since LHS = RHS, P(1) is true.
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Inductive Hypothesis:
Assume P(m) is true for some positive integer m.
That is, assume 1+2+3+⋯+m=2m(m+1).
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Inductive Step:
We need to prove that P(m+1) is true.