In the eighth grade, I practiced on page (1) 10 of Beijing Normal University.
1. Solution (1)(4) The length of three sides of a right triangle can be taken; (2)(3) cannot be used as the length of three sides of a right triangle.
Reason: (1)9? + 12? = 15? , (4) 15? +36? =39? All satisfy the trilateral relationship of right triangle;
(2) 12? + 1822? , (3) 12? +3536? , does not satisfy the trilateral relationship of a right triangle.
2. There are four solutions, in which △ Abe, △ BEF, △ BCF and △ DEF are right triangles.
The verification that △BEF is a right triangle is as follows: According to Pythagorean theorem, BE? =20，EF？ =5，BF？ =BC？ +CF？ = 16+9=25, which means BE? +EF？ =BF？ So △BEF is a right triangle.
Which of the other three triangles? First,? c，？ D is a right angle.
Answers to Grade 8 Mathematics Book of Beijing Normal University Edition (2) Page 29
Answers to the eighth grade mathematics books published by Beijing Normal University (3)