3＜√13＜4
-√13的整数部分为-4
9-√13的整数部分m = 9-4=5
9-√13的小数部分n = 9-√13-5 = 4-√13
2mn-n^2 = n(2m-n) = (4-√13){2*5-(4-√13) = (4-√13){6+√13) = 11-2√13