## Elementary Geometry for College Students (7th Edition)

$a=\sqrt{b^{2}+c^{2}}$
The length of AB can be calculated: $x_{B}-x{A}=a$ The length of AD can be calculated with the Pythagorean Theorem: $\sqrt{(x_{d}-x_{a})^{2}+(y_{d}-y_{a})^{2}} = \sqrt{(b-0)^{2}+(c-0)^{2}}$ So, $a=\sqrt{b^{2}+c^{2}}$