Imagining Imaginary Numbers

Algebra Level 4

i 63 + i 62 + i 61 + + i 3 + i 2 + i + 1 i 3 + i 2 + i + 1 = ? \large \frac{i^{63}+i^{62}+i^{61}+\ldots + i^{3}+i^{2}+i +1}{i^3+i^2+i+1} = \ ?

Undefined 32 32 Does not exist Zero Indeterminate 16 16 None of the options

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2 solutions

Efren Medallo
Jun 12, 2015

Let x x be the answer to the expression.

Factoring the terms, we get

x = ( i 32 + 1 ) ( i 16 + 1 ) ( i 8 + 1 ) ( i 4 + 1 ) ( i 2 + 1 ) ( i + 1 ) ( i 2 + 1 ) ( i + 1 ) \large x = \frac{(i^{32}+1)(i^{16}+1)(i^{8}+1)(i^{4}+1)(i^2+1)(i+1)}{(i^{2}+1)(i+1)}

Now, we can cancel the term i + 1 i + 1 , since its nonzero, but not i 2 + 1 i^2 +1 because it has a value of zero.

We see now that x = 0 0 x = \frac {0}{0} .

This implies that x × 0 = 0 x \times 0 = 0 . Since any value of x x would make the statement correct, this makes it indeterminate .

PS: x x will only be undefined if x = a 0 x = \frac {a}{0} for nonzero value of a a .

Also, we cannot use L'Hopital's rule here because i i is a number, and not a variable. After all, we're not looking for limits.

why won't we cut out the factors

Himanshu Tuteja - 6 years ago
Arulx Z
Jul 6, 2015

= 1 + i + i 2 + i 3 + i 4 i 63 1 + i + i 2 + i 3 = 1 + i + i 2 + i 3 + i 4 i 63 1 + i + 1 i = 1 + i + i 2 + i 3 + i 4 i 63 0 \huge{=\frac { 1+i+{ i }^{ 2 }+{ i }^{ 3 }+{ i }^{ 4 }\dots { i }^{ 63 } }{ 1+i+{ i }^{ 2 }+{ i }^{ 3 } } \\ =\frac { 1+i+{ i }^{ 2 }+{ i }^{ 3 }+{ i }^{ 4 }\dots { i }^{ 63 } }{ 1+i+1-i } \\ =\frac { 1+i+{ i }^{ 2 }+{ i }^{ 3 }+{ i }^{ 4 }\dots { i }^{ 63 } }{ 0 } }

Since the denominator is 0, we can immediately see that the fraction's value will be indeterminate. Hence there is no need to evaluate expression in the numerator.

Moderator note:

It would be made much clearer if you immediately explained that the denominator is 0, hence we have an indeterminate form.

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