13.14.-1/715. (1,5/4)16.

17. solution: a/sinA=b/sinB is known, so it can be known from the title that Sina+sinb = sinacota+sinbcottb = COSA+cosb.

There is SINA-COSA = COSB-SINB (*), and the squares on both sides are sin2A=sin2B.

And if a and b are internal angles of a triangle, then A=B( 1) or a+b = 90 "(2).

When it is (1), (*) must have A=B=45 "and A+B=90".

To sum up, we know that C=90 "

18. Solution:) Know that p=0.5*0.5+2*0.5*0.5*0.3=0.4.

(! ! ) x obeys binomial distribution, and the distribution table is omitted, Ex=np= 1.6.

19. This topic should use the method of vector coordinate system.

(! ) a little proof

(! ! ) 150"

20. Solution: (! ) f' (x) = lnx+ 1/x, so xf' (x) = xlnx+ 1

+1 (x. >0), variable separation has a "lnx-x,

A' = 1/x-1, the maximum number of plants when x = 1 is obtained, and the range of a is (-1,+infinity).

(! ! ) This problem uses the second derivative. Firstly, it is proved that f(x) is always increasing function in the definition domain and is known.

If f( 1) is 0, the relationship between x and 1 (less than, greater than and equal to) needs to be discussed.

2 1*……%…*…(…………%…%…￥—

22. The problem shows an+1-2 =1/2-1/an.

Then1/(an+1-2) = 2an/(an-2), that is

Bn+ 1 = 2Bn *( 1/Bn+2)= 2+4Bn

Construct a new function, add 2/3 to both sides, and a geometric series will appear with {Bn+2/3} as the first term, the common ratio-1/3 as 4, and BN =- 1/3 *.

(! ! )* * * & amp……%￥@#