Round flowerbed

I will make a sidewalk up to 0.5 meters wide around a round flowerbed with a diameter of 6 meters. How much gravel is needed if the layer is 5 cm high?

Correct answer:

V =  0.5105 m³

Step-by-step explanation:

$$\begin{gathered} D=6\text{m} \\ h=0.5\text{m} \\ c=5\text{cm}\to \text{m}=5:100\text{m}=0.05\text{m} \\ r=D/2=6/2=3\text{m} \\ {r}_2=r+h=3+0.5=\frac{7}{2}=3.5\text{m} \\ S=\pi \cdot r_2^2-\pi \cdot r^2=3.1416\cdot 3.5^2-3.1416\cdot 3^2\doteq 10.2102{\text{m}}^2 \\ V=S\cdot c=10.2102\cdot 0.05=0.5105{\text{m}}^3 \end{gathered}$$

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