### Instruction: Evaluate $|1-4i|$.### Response: To evaluate the absolute value of a complex number, I need to use the formula $|a+bi|=\sqrt{a^2+b^2}$, where $a$ and $b$ are the real and imaginary parts, respectively.
In this case, $a=1$ and $b=-4$, so I can plug them into the formula and simplify:
\begin{align*}
|1-4i|&=\sqrt{1^2+(-4)^2}\\
&=\sqrt{1+16}\\
&=\sqrt{17}
\end{align*}