Complex on Complex

Algebra Level 5

Suppose that:

( 2 + i 2 3 ) ( 1 + i ) = α + i β \large \left(\sqrt{2} + i\dfrac{\sqrt{2}}{3}\right)^{(1+i)} = \alpha + i \beta

where, α \alpha and β \beta are real numbers and i 2 = 1 i^2 = - 1 . Find 1000 α + 1000 β \lfloor 1000\alpha \rfloor + \lfloor 1000\beta \rfloor , where x \lfloor x \rfloor is the greatest integer function.


The answer is 1524.

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2 solutions

Let x x be that expression, so using the exponential function we have:

x = e ( 1 + i ) ln ( 2 + i 2 3 ) x=e^{(1+i)\ln\left(\sqrt{2} + i\frac{\sqrt{2}}{3}\right)}

Now, use the formula for the principal logarithm of a complex number:

ln ( z ) = ln ( z ) + i arg ( z ) ln ( 2 + i 2 3 ) = ln ( 2 5 3 ) + i arctan ( 1 3 ) \ln(z)=\ln(|z|)+i\text{arg}(z) \\ \ln\left(\sqrt{2} + i\dfrac{\sqrt{2}}{3}\right)=\ln\left(\dfrac{2\sqrt{5}}{3}\right)+i \arctan\left(\dfrac{1}{3}\right)

So, we have now:

x = e ( 1 + i ) ( ln ( 2 5 3 ) + i arctan ( 1 3 ) ) x=e^{(1+i)\left(\ln\left(\frac{2\sqrt{5}}{3}\right)+i \arctan\left(\frac{1}{3}\right)\right)}

Expand:

x = e ln ( 2 5 3 ) arctan ( 1 3 ) + i ( ln ( 2 5 3 ) + arctan ( 1 3 ) ) x = e ln ( 2 5 3 ) arctan ( 1 3 ) [ cos ( ln ( 2 5 3 ) + arctan ( 1 3 ) ) + i sin ( ln ( 2 5 3 ) + arctan ( 1 3 ) ) ] x=e^{\ln\left(\frac{2\sqrt{5}}{3}\right)-\arctan\left(\frac{1}{3}\right)+i\left(\ln\left(\frac{2\sqrt{5}}{3}\right)+\arctan\left(\frac{1}{3}\right)\right)} \\ x=e^{\ln\left(\frac{2\sqrt{5}}{3}\right)-\arctan\left(\frac{1}{3}\right)} \left[\cos\left(\ln\left(\frac{2\sqrt{5}}{3}\right)+\arctan\left(\frac{1}{3}\right)\right)+i\sin \left(\ln\left(\frac{2\sqrt{5}}{3}\right)+\arctan\left(\frac{1}{3}\right)\right)\right]

To evaluate that expression, we need to use radians:

x 1.08058579159 ( 0.75114306241 + 0.6601394548 i ) x 0.81167452069 + 0.71333731532 i x \approx 1.08058579159(0.75114306241+0.6601394548i) \\ x \approx 0.81167452069+0.71333731532i

Comparing we get 1000 α = 811 \lfloor 1000\alpha \rfloor=811 and 1000 β = 713 \lfloor 1000\beta \rfloor=713 , so the final answer is 811 + 713 = 1524 811+713=\boxed{1524} .

First we write ( 2 + i 2 3 ) (\sqrt {2} +i \dfrac { \sqrt {2} } {3} ) in form k ( cos a + i sin a ) k(\cos{a}+i\sin{a}) .

X = ( 2 + i 2 3 ) 1 + i = ( 2 5 3 ) 1 + i ( 3 2 2 5 + i 1 10 ) 1 + i X={ ( \sqrt {2} +i \dfrac { \sqrt {2} } {3} ) }^{ 1+i }={ (\frac { 2\sqrt { 5 } }{ 3 } ) }^{ 1+i }{ (\frac { 3 }{ 2 } \sqrt { \frac { 2 }{ 5 } } +i\frac { 1 }{ \sqrt { 10 } } ) }^{ 1+i }

Now we can write both ( 2 5 3 ) (\frac { 2\sqrt { 5 } }{ 3 } ) and ( 3 2 2 5 + i 1 10 ) (\frac { 3 }{ 2 } \sqrt { \frac { 2 }{ 5 } } +i\frac { 1 }{ \sqrt { 10 } } ) in their exponential form. By separating real and imaginary numbers in exponent we can get one purely real number and one complex.

X = e ( 1 + i ) ln 2 5 3 e ( 1 + i ) i arcsin 1 10 = e ln 2 5 3 arcsin 1 10 e i ( ln 2 5 3 + arcsin 1 10 ) \large X={ e }^{ (1+i)\ln { \frac { 2\sqrt { 5 } }{ 3 } } }{ e }^{ (1+i)i\arcsin { \frac { 1 }{ \sqrt { 10 } } } }={ e }^{ \ln { \frac { 2\sqrt { 5 } }{ 3 } } -\arcsin{ \frac { 1 }{ \sqrt { 10 } } } }{ e }^{ i(\ln { \frac { 2\sqrt { 5 } }{ 3 } } +\arcsin { \frac { 1 }{ \sqrt { 10 } } } ) }

This can easily be transformed to the form of α + i β \alpha+i\beta .

X = e ln 2 5 3 arcsin 1 10 [ cos ( ln 2 5 3 + arcsin 1 10 ) + i sin ( ln 2 5 3 + arcsin 1 10 ) ] X={ e }^{ \ln { \frac { 2\sqrt { 5 } }{ 3 } } -\arcsin { \frac { 1 }{ \sqrt { 10 } } } } \quad[ \cos { (\ln { \frac { 2\sqrt { 5 } }{ 3 } } +\arcsin { \frac { 1 }{ \sqrt { 10 } } )+i\sin { (\ln { \frac { 2\sqrt { 5 } }{ 3 } } +\arcsin{ \frac { 1 }{ \sqrt { 10 } } ) } } } }]

α = e ln 2 5 3 arcsin 1 10 cos ( ln 2 5 3 + arcsin 1 10 ) 0 , 811674 \alpha={ e }^{ \ln { \frac { 2\sqrt { 5 } }{ 3 } } -\arcsin { \frac { 1 }{ \sqrt { 10 } } } } \quad \cos { (\ln { \frac { 2\sqrt { 5 } }{ 3 } } +\arcsin { \frac { 1 }{ \sqrt { 10 } } ) } }\approx0,811674

β = e ln 2 5 3 arcsin 1 10 sin ( ln 2 5 3 + arcsin 1 10 ) 0 , 713337 \beta={ e }^{ \ln { \frac { 2\sqrt { 5 } }{ 3 } } -\arcsin { \frac { 1 }{ \sqrt { 10 } } } } \quad \sin{ (\ln { \frac { 2\sqrt { 5 } }{ 3 } } +\arcsin { \frac { 1 }{ \sqrt { 10 } } ) } }\approx0,713337

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