# Math Insight

### Applet: Ordinary derivative of heating cost

Imagine the function $h(T)$ shown as a think green line was the cost of heating your house given the outside temperature $T$. The derivative $dh/dT$ is calculated at the temperature $T=a$. You can change $a$ by dragging the red point at $(a,h(a))$ with your mouse. The thin blue line is tangent to the graph of $h(T)$ at the point $T=a$, and its slope is the derivative $dh/dT$.

Applet file: ordinary_derivative_heating_cost.ggb