To find the radius of the original circle before dilation, we can use the scale factor of 1/5. If the radius of the dilated circle is 18 cm, we can find the radius of the original circle (let's call it \( r \)) using the formula:
\[ r \times \text{scale factor} = \text{dilated radius} \]
In this case, the scale factor is 1/5, and the dilated radius is 18 cm. Thus, we have:
\[ r \times \frac{1}{5} = 18 \]
To solve for \( r \), we can multiply both sides of the equation by 5:
\[ r = 18 \times 5 \] \[ r = 90 , \text{cm} \]
So, the radius of the original circle you dilated is 90 cm.