Test and analysis: the area of the big square is 13, and the area of the small square is 1, so that the area of each small right triangle is 3, which can be obtained, and then the result can be obtained according to the complete square formula.

Judging from the meaning of the question, then

therefore

So choose C.

Comments: This topic is called Basic Application Problem, and students can only complete it by mastering Pythagorean Theorem and completing the square formula.