How do you do #18 in Test 6 Section 3 without a calculator?
The key to this one is recognizing that you’ve got similar triangles. First, both triangle CDB and triangle CEA contain angle C. The question also tells you that 
, which means that angle CDB is also a right angle. Therefore, you have angle-angle similarity.
It’s also good to recognize a 6-8-10 triangle quickly. Know your Pythagorean triples! 🙂
From there, all you need to do is recognize that the length of 
is 3 times the length of 
, which means all the sides of triangle CEA are 3 times the length of their corresponding sides in triangle CDB. Therefore, because the length of 
is 10, the length of 
must be 30.