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If x + (1/x) = 3√2, then what is the value of x5 + (1/x5)?
178√3
789√2
1581√2
717√2
Let’s break it down step by step:
- You’re given x + (1/x) = 3v2.
- The key is to build up higher powers recursively.
- Start with the basic formula:
(x + 1/x) = a
(x² + 1/x²) = a² – 2
(x³ + 1/x³) = a³ – 3a
and so on.
- For a = 3v2:
x + 1/x = 3v2
x² + 1/x² = (3v2)² – 2 = 18 – 2 = 16
x³ + 1/x³ = (3v2)³ – 3*(3v2) = 54v2 – 9v2 = 45v2
x4 + 1/x4 = (x² + 1/x²)² – 2 = 16² – 2 = 254
x5 + 1/x5 = (x4 + 1/x4)*(x + 1/x) – (x³ + 1/x³)
= 254*(3v2) – 45v2
= (254×3 – 45)v2
= (762 – 45)v2
= 717v2
- Putting it together:
- Option 1: 178v3 (not our answer)
- Option 2: 789v2 (not our answer)
- Option 3: 1581v2 (not our answer)
- Option 4: 717v2 (this one matches perfectly!)
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