Answer:
addition
Step-by-step explanation:
[tex]EC=\sqrt{10^{2}-8^{2} } =\sqrt{36} =6\\\\ED=EC+CD = 6+9=15\\\\AD=\sqrt{EA^{2}+ED^{2} } =\sqrt{8^{2}+15^{2} } =\sqrt{289} =17[/tex]
