1. The problem asks to explain why the function in 1d is decreasing. 2. A function is decreasing on an interval if for any two points $x_1$ and $x_2$ in that interval, whenever $x_1 < x_2$, then $f(x_1) > f(x_2)$. 3. To determine if a function is decreasing, we often look at its derivative $f'(x)$. 4. If $f'(x) < 0$ for all $x$ in the interval, then the function is decreasing there. 5. For 1d, calculate the derivative $f'(x)$ and check its sign. 6. If $f'(x)$ is negative throughout the domain of interest, this confirms the function is decreasing. 7. This means as $x$ increases, $f(x)$ decreases, which is the definition of a decreasing function. 8. Therefore, 1d is decreasing because its derivative is negative on the interval considered.