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If a is positive and a² + 1/a² = 7, then a³ + 1/a³ equals:
A21
B3√7
C18
D7√7
Answer & Solution
Correct answer: C. 18
1. (a + 1/a)² = a² + 1/a² + 2 = 7 + 2 = 9, so a + 1/a = 3.
2. a³ + 1/a³ = (a + 1/a)³ − 3(a + 1/a).
3. = 27 − 9 = 18.
_Source: RRB Group D CEN-02/2018 PYQ, Q.39._