DCACB

1 1.90

12. 1

13.20/3

14.- 1/2 or 1

15.3/5

16( 1)60 degrees

(2)3/2 of the root number of the third degree

17.( 1) A

(2)7/ 10

18.( 1)an=2n- 1

(2)n= 1。 Sn=9

N > 1。 Sn = 2 to the n+n power.

19.(2) 1/8

-

If satisfied, I am willing to adopt O(∩_∩)O~