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nexus9112 [7]
3 years ago
7

Simplify: 1 + i + i^2 + i^3 +i^4 +i^5 if i^2 =-1.

SAT
2 answers:
Aloiza [94]3 years ago
7 0

Answer: 1 + i

Explanation:

Vika [28.1K]3 years ago
3 0

1 + <em>i</em> + <em>i</em> ² + <em>i</em> ³ + <em>i</em> ⁴ + <em>i</em> ⁵ = 1 + <em>i</em> + <em>i</em> ² (1 + <em>i</em> + <em>i </em>² + <em>i</em> ³)

… = 1 + <em>i </em>+ <em>i</em> ² (1 + <em>i</em> + <em>i</em> ² (1 + <em>i</em> ))

… = 1 + <em>i</em> + (-1) (1 + <em>i</em> + (-1) (1 + <em>i</em> ))

… = 1 + <em>i</em> + (-1) (1 + <em>i</em> - 1 - <em>i</em> )

… = 1 + <em>i</em>

<em />

Alternatively, notice that

1 - <em>i</em> ⁶ = (1 - <em>i</em> ) (1 + <em>i</em> + <em>i</em> ² + <em>i</em> ³ + <em>i</em> ⁴ + <em>i</em> ⁵)

so that

1 + <em>i</em> + <em>i</em> ² + <em>i</em> ³ + <em>i</em> ⁴ + <em>i</em> ⁵ = (1 - <em>i</em> ⁶) / (1 - <em>i</em> )

Now, <em>i</em> ⁶ = (<em>i</em> ²)³ = (-1)³ = -1, so

1 + <em>i</em> + <em>i</em> ² + <em>i</em> ³ + <em>i</em> ⁴ + <em>i</em> ⁵ = (1 - (-1)) / (1 - <em>i</em> )

… = 2 / (1 - <em>i</em> )

Multiply the numerator and denominator by the conjugate of 1 - <em>i</em> :

… = 2 (1 + <em>i</em> ) / ((1 - <em>i</em> ) (1 + <em>i</em> ))

… = 2 (1 + <em>i</em> ) / (1² - <em>i</em> ²)

… = 2 (1 + <em>i</em> ) / (1 + 1)

… = 1 + <em>i</em>

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