1 is a triangle. According to Pythagorean theorem, dc= root number 5, ce=2 times root number 5, and point D is df vertical be, which means that df=4, bf= 1, dfe is a right triangle, and fe=3, so de=5. According to Pythagorean theorem, dce is a right triangle.

I can't help you because I don't have a textbook.