Problem 135
Easy Powers and Roots

Write cot((π)/(4)-iln2) in the form u+iv

Write $\cot\left(\dfrac{\pi}{4}-i\ln2\right)$ in the form $u+iv$.

Explanation of Method

Use $\cot(x+iy)=\dfrac{\sin(2x)-i\sinh(2y)}{\cosh(2y)-\cos(2x)}$, and evaluate the hyperbolic terms exactly using $e^{\ln2}=2$.

Step 1 · Apply with $x=\frac{\pi}4,\ y=-\ln2$

$$ \cot\left(\frac\pi4-i\ln2\right)=\frac{\sin\left(\frac\pi2\right)-i\sinh(-2\ln2)}{\cosh(-2\ln2)-\cos\left(\frac\pi2\right)}=\frac{1+i\sinh(2\ln2)}{\cosh(2\ln2)} $$

(using $\sin\frac\pi2=1,\ \cos\frac\pi2=0$, and $\sinh,\cosh$ odd/even.)

Step 2 · Evaluate $\sinh(2\ln2)$ and $\cosh(2\ln2)$ exactly

Since $e^{2\ln2}=2^2=4$ and $e^{-2\ln2}=2^{-2}=\frac14$:

$$ \sinh(2\ln2)=\frac{4-\frac14}2=\frac{15}8, \qquad \cosh(2\ln2)=\frac{4+\frac14}2=\frac{17}8 $$

Step 3 · Substitute and simplify

$$ \cot\left(\frac\pi4-i\ln2\right)=\frac{1+i\frac{15}8}{\frac{17}8}=\frac{8+15i}{17}=\frac8{17}+\frac{15}{17}i $$

Step 4 · Result

$$ \boxed{u=\frac8{17}, \qquad v=\frac{15}{17}} $$

Verification

Direct numerical evaluation of $\cot(0.785398-0.693147i)$ gives $0.470588\ldots+0.882353\ldots i$, which matches $\frac8{17}\approx0.470588$ and $\frac{15}{17}\approx0.882353$ exactly.

Result

$$ \cot\left(\frac\pi4-i\ln2\right)=\frac8{17}+\frac{15}{17}i $$

An unusually clean result — the use of $\ln2$ as the imaginary part makes $e^{\pm2\ln2}$ rational, so the $8$–$15$–$17$ Pythagorean triple falls straight out of the hyperbolic evaluation.