Break the limit

Algebra Level 5

sin ( z ) cos ( z ) = 7 \large \sin{(z)}\cos{(z)} = 7

If the solutions of the equation above is of the form π a i b ln ( c ± d ) + n π \dfrac{\pi}{a}-\dfrac{i}{b}\ln(c\pm \sqrt{d})+{n}\pi where a a , b b , c c and d d are positive integers. Find a + b + c + d a+b+c+d .


The answer is 215.

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2 solutions

Mark Hennings
Jul 30, 2017

We need to solve sin 2 z = 14 \sin2z = 14 . Writing 2 z = x + i y 2z=x+iy we deduce that sin x cosh y + i cos x sinh y = 14 \sin x \cosh y + i\cos x \sinh y \; = \; 14 so we need cos x = 0 , sin x = 1 \cos x = 0, \sin x = 1 and cosh y = 14 \cosh y = 14 . Thus x = ( 2 n + 1 2 ) π x = (2n+\tfrac12)\pi for integer n n , and y = ln ( 14 + 195 ) = ln ( 14 ± 195 ) y = \mp\ln(14+\sqrt{195}) = -\ln(14\pm\sqrt{195}) . Thus z = 1 2 ( x + i y ) = n π + 1 4 π 1 2 i ln ( 15 ± 195 ) z \; = \; \tfrac12(x+iy) \; =\; n\pi + \tfrac14\pi - \tfrac12i\ln(15\pm\sqrt{195}) for integer n n , making the answer 4 + 2 + 14 + 195 = 215 4+2+14+195 = \boxed{215} .

Marco Brezzi
Jul 30, 2017

Rewriting s i n ( z ) sin(z) and c o s ( z ) cos(z) we get

e i z e i z 2 i e i z + e i z 2 = 7 e 2 i z e 2 i z 4 i = 7 \dfrac{e^{iz}-e^{-iz}}{2i}\cdot\dfrac{e^{iz}+e^{-iz}}{2}=7 \iff \dfrac{e^{2iz}-e^{-2iz}}{4i}=7

Multiply both sides of the equation by 4 i e 2 i z 4ie^{2iz} to get a quadratic in e 2 i z e^{2iz}

( e 2 i z ) 2 28 i e 2 i z 1 = 0 (e^{2iz})^2-28ie^{2iz}-1=0

Solving for e 2 i z e^{2iz}

e 2 i z = 14 i ± ( 14 i ) 2 + 1 = 14 i ± 195 = i ( 14 ± 195 ) e^{2iz}=14i\pm\sqrt{(14i)^2+1}=14i\pm\sqrt{-195}=i(14\pm\sqrt{195})

Taking the natural logarithm of both sides and using the fact that l o g ( a b ) = l o g ( a ) + l o g ( b ) log(ab)=log(a)+log(b)

2 i z = l n [ i ( 14 ± 195 ) ] = l n ( i ) + l n ( 14 ± 195 ) 2iz=ln[i(14\pm\sqrt{195})]=ln(i)+ln(14\pm\sqrt{195})

To calculate l n ( i ) ln(i) we express i i in its polar form

l n ( i ) = l n ( e i π 2 + 2 π n i ) = i ( π 2 + 2 π n ) n Z ln(i)=ln(e^{i\frac{\pi}{2}+2\pi ni})=i\left(\frac{\pi}{2}+2\pi n\right) \qquad n\in\mathbb{Z}

Substituting back and solzing for z z

z = 1 2 i [ i ( π 2 + 2 π n ) + l n ( 14 ± 195 ) ] = π 4 i 2 l n ( 14 ± 195 ) + π n z=\dfrac{1}{2i}\left[i\left(\frac{\pi}{2}+2\pi n\right)+ln(14\pm\sqrt{195})\right]=\dfrac{\pi}{4}-\dfrac{i}{2}ln(14\pm\sqrt{195})+\pi n

Therefore the solution is

4 + 2 + 14 + 195 = 215 4+2+14+195=\boxed{215}

Solution is okay but there is a problem with writing conventions. when you are writing ln(i) it means the principal value of the complex logarith and it is directly equal to iπ/2 and if you want to find the general value write it as Log(i) (with capital L) . Now Log(i) = 2nπ + iπ/2 .

Arghyadeep Chatterjee - 2 years, 9 months ago

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