Complex Log

Algebra Level 4

Real numbers a a and b b are such that sin ( ln i i ) = a + i b \sin \left( \ln i^i \right) = a+ib . Find a + b a+b .

Note: We are taking the standard principal branch of the complex logarithm.


The answer is -1.

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2 solutions

Viki Zeta
Oct 8, 2016

sin ( ln ( i i ) ) = ? e i π = 1 e i π 2 = i ( e i π 2 ) i = i i sin ( ln ( i i ) ) = sin ( ln ( e i π 2 ) i ) = sin ( i ln ( e i π 2 ) ) = sin ( i i π 2 ln ( e ) ) = sin ( 1 × π 2 ) = sin ( π 2 ) = 1 1 = a + i b a = 1 , b = 0 a + b = 1 \displaystyle \sin(\ln(i^i)) = ? \\ \displaystyle e^{i\pi} = -1 \\ \displaystyle e^{\dfrac{i\pi}{2}} = i\\ \\ \displaystyle \left(e^{\dfrac{i\pi}{2}}\right)^i = i^i \\ \displaystyle \sin(\ln(i^i)) = \sin\left(\ln\left(e^{\dfrac{i\pi}{2}}\right)^i\right) \\ \displaystyle = \sin\left(i\ln\left(e^{\dfrac{i\pi}{2}}\right)\right) \\ \displaystyle = \sin\left(i\dfrac{i\pi}{2}\ln\left(e\right)\right)\\ \displaystyle = \sin\left(-1 \times \dfrac{\pi}{2}\right) \\ \displaystyle = - \sin\left(\dfrac{\pi}{2}\right) \\ \displaystyle = -1 \\ \displaystyle \boxed{\therefore -1 = a + ib \implies a = -1, b = 0 \implies a + b = -1}

Steven Chase
Oct 8, 2016

s i n sin l o g i i log i^i = s i n sin l o g ( e i π 2 ) i log (e^{i \frac{\pi}{2}})^i = s i n sin l o g ( e π 2 ) log (e^{- \frac{\pi}{2}}) = s i n ( π 2 ) sin(-\frac{\pi}{2}) = 1 -1

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