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9. Calculați:
a) (−1)¹ + (−1)² + (−1)³ + ... + (−1)2⁹;
b) (-1)¹. 1 + (−1)² · 2 + (-1)³3+ ... + (-1)3⁰.30;
c) (−1)¹ · (−1) + (−1)² · (-2) + (−1)³ · (-3) + ... + (−1)2⁹ · (-29).