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Let R = (A, B) and S = (A,C), and let r (R) and s(S) be relations.Write
expressions in relational algebra for each of the following queries:
a. {< a > | ∃ b (< a, b > ∈ r ∧ b = 7)}
b. {< a, b, c > | < a, b > ∈ r ∧ < a, c > ∈ s}
c. {< a > | ∃ c (< a, c > ∈ s ∧ ∃ b1, b2 (< a, b1 > ∈ r ∧ < c, b2 > ∈ r ∧ b1 > b2))}
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