1. The problem asks: If you know the diameter of a circle, how can you find the area? 2. Recall the formula for the area of a circle: $$A = \pi r^2$$ where $r$ is the radius. 3. The diameter $d$ is twice the radius: $$d = 2r \implies r = \frac{d}{2}$$ 4. Substitute $r = \frac{d}{2}$ into the area formula: $$A = \pi \left(\frac{d}{2}\right)^2$$ 5. Simplify the expression: $$A = \pi \frac{d^2}{4}$$ 6. So, the area of a circle when you know the diameter $d$ is: $$\boxed{A = \frac{\pi d^2}{4}}$$ This means you square the diameter, multiply by $\pi$, and then divide by 4 to get the area.