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  1. 7n-2 ∈ O(n)
    need c>0 and n0≥1 such that 7n-2 ≤ c·n for n≥n0
    ⇒  true for c=7 and n0=1
  2. 3n3 + 20n2 + 5 ∈ O(n3)
    need c>0 and n0≥1 such that 3n3+20n2+5 ≤ c·n3 for n≥n0
    ⇒  true for c=4 and n0=21
  3. 3·log n + 5 ∈ O(log n)
    need c>0 and n0≥1 such that 3·log2 n+5 ≤ c·log n for n≥n0
    ⇒  true for c=8 and n0=2    (can also choose c=4, along with which n0?)