When calculating the diameter equivalent ($\text{de}$) of a round duct in a HVAC system, and we know the rectangular duct size, we can use the following formula:
$\text{de} = 1.30 \times \frac{(a \times b)^{0.625}}{(a + b)^{0.25}}$
BUT IF we know $\text{de}$, and we know $a$, what formula could be used to solve for $b$? For testing purposes, I am assuming $\text{de}$ is 18" and $a$ is 16".
Using WolframAlpha...I was able to achieve an answer but they graphed to achieve the solution by using the intersecting lines...but I would like to solve this without graphing.