点为切点，如图4. .

于是 .解处 .

的值为1或4或5.6.······························································································ 10分

26.解：探究：12，15，84·························································································· 3分

拓展：(1)由三角形面积公式，得 .······························· 4分

(2)由(1)得，

.············································································ 5分

由于边上的高为，

的取值范围是 .

随的增大而减小，

当时，的最大值为15.······································································ 7分

当时，的最小值为12.··········································································· 8分

(3)的取值范围是或 .······························································ 10分

发现：所在的直线，···························································································· 11分

最小值为 .············································································································· 12分