恩，或许有同事或者同学会有用。先上效果图

完成这个动作很麻烦，先要延长到交点，然后再打断，然后再变换图层，才能正常标注，很伤脑筋。。。于是求助某论坛做了个程序。

;绘制圆弧切线
(defun c:tt (/ last_obj last_se obj obj_data obj_data_list obj_i obj_name n plst ss var1 var2 )
  (princ "\n<----圆弧切线---->")
  (princ "\n请选择圆弧,多段线/<退出>...")
  (setq var1 (getvar "cmdecho"))
  (setq var2 (getvar "osmode"))
  (setvar "cmdecho" 0)
  (setvar "osmode" 0)
  (setq n 0)
  (setq obj (ssget '((0 . "*POLYLINE,ARC"))))
  (if obj
    (repeat (sslength obj)
      (setq ent (entget (ssname obj n)))
      (if (= (cdr (assoc 0 ent)) "ARC")
        (progn
                (setq plst (sys_get_pt_arc_tan ent))
          (command "_.line" (car plst)  (caddr plst) "")
          (command "_.line" (cadr plst) (caddr plst) "")
        )
      )
      (if (= (cdr (assoc 0 ent)) "LWPOLYLINE")
              (progn
                (setq last_obj (ssget "p"))
                (setq obj_data_list '())
          (setq ss (ssadd (ssname obj n)))
                (command "_undo" "mark")
                (command "_explode" ss)
                (setq last_se (ssget "p"))
                (if (not (eq last_se last_obj))
                  (progn
                    (setq obj_i -1)
                    (repeat (sslength last_se)
                            (setq obj_name (ssname last_se (setq obj_i (1+ obj_i))))
                            (setq obj_data (entget obj_name))
                            (if (= (cdr (assoc 0 obj_data)) "ARC")
                              (setq obj_data_list (append (list obj_data) obj_data_list))
                            )
                    )
                  )
                )
                (command "_undo" "back")
                (if obj_data_list
                  (foreach i obj_data_list
                          (setq plst (sys_get_pt_arc_tan i))
              (command "_.line" (car plst)  (caddr plst) "")
              (command "_.line" (cadr plst) (caddr plst) "")
                  )
                )
              )
      )
      (setq n (+ n 1))
    )
  )
  (setvar "cmdecho" var1)
  (setvar "osmode" var2)
  (princ)
)
(defun sys_get_pt_arc_tan (ent / cen jdq jdz p1 p2 p3 p4 p5 rad)
  (setq cen (cdr (assoc 10 ent)));获取圆弧圆心
  (setq rad (cdr (assoc 40 ent)));获取圆弧半径
  (setq jdq (cdr (assoc 50 ent)));获取起始角度
  (setq jdz (cdr (assoc 51 ent)));获取终止角度
  (setq p1 (polar cen jdq rad);获取圆弧起始点
        p2 (polar cen jdz rad);获取圆弧终止点
        p3 (polar p1 (+ (/ pi 2) jdq) rad);获取起始点切线方向的半径点
        p4 (polar p2 (+ (/ pi 2) jdz) rad);获取终止点切线方向的半径点
        p5 (inters p1 p3 p2 p4 nil);切线交点
  )
  (if (not p5)
    (princ "\n切线无交点...")
    (list p1 p2 p5)
  )
)

使用办法：

1.复制代码保存为tt.lsp，存到某个地方；

2.工具-自定义-界面-lisp文件上右键选择加载，确定

3.敲入tt命令使用，可框选，选则圆弧即可

顺便，常用的还有个打断于点，这个有图标命令，但是木有快捷键，两只手总比一只手有速度，于是自己做了一个：

;打断于点
(defun c:dd ()
(command "_break" pause "f" pause "@")
)