In the following, directly cite any and all properties of the real numbers that are used in your solutions.
(a) Prove or disprove: for all nonzero a ∈ R, there exists a unique a−1 ∈ R such that aa−1 = 1. (Note: M4 only gives existence of a−1, not uniqueness).
(b) Prove or disprove: for all a, b, c, d ∈ R , if a ≤ b and c ≤ d, then ac ≤ bd.
(c) Prove or disprove: ||a| − |b|| ≤ |a − b| for all a, b ∈ R
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