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What is the value of the expression `ilog(x-i)` `+ i^2pi` `+ i^3log(x+i)`...

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user8235304 | Student, Grade 11 | (Level 1) Valedictorian

Posted August 21, 2013 at 6:17 AM via web

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What is the value of the expression `ilog(x-i)` `+ i^2pi` `+ i^3log(x+i)` `+i^4(2tan^(-1)x)` , `x>0`? Is it

A) 0

B) 1

C) 2

or D) 3 ?

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mathsworkmusic | (Level 1) Educator

Posted August 21, 2013 at 2:39 PM (Answer #1)

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We want the value of the expression

`ilog(x-i) + i^2pi + i^3log(x+i) + i^4(2tan^(-1)x)`

given that `x>0`.

Firstly consider the complex inverse tangent function

`tan^(-1)z = i/2[log(1-iz) -log(1+iz)]`

By simple algebraic manipulation (using the fact that `i^2=-1` and laws of logarithms) we can rewrite this as

`tan^(-1)z = i/2[log((z+i)/i) - log((z-i)/(-i))]`

`= i/2[log(z+i) - log(z-i) - log(i) + log(-i)]`

Therefore

`2tan^(-1)x = ilog(x+i) - ilog(x-i) - ilog(i) + ilog(-i)`

Noting that `i^3 = i(i^2) = -i` and further that `i^4 = i(i^3) = -i^2 = 1`  we see that ` ` the term `i^(4)2tan^(-1)x` in the original expression cancels out the ` ` logarithmic terms in that expression.

We are left with finding the value of

`i^2pi - ilog(i) + ilog(-i)`

Now, writing `i` and `-i` in polar coordinates on the complex plane, as

`i = e^(i(pi/2))`

`-i = e^(-i(pi/2))`

gives us that `log(i) = i(pi/2)` and `log(-i) = -i(pi/2)` .

Therefore, `-ilog(i) +ilog(-i) = -2i^2(pi/2) = 2(pi/2) = pi`

Finally, the expression

`i^2pi - ilog(i) + ilog(-i)`  equates to (`-pi + pi `), ie zero.

The answer  is A) 0

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