请教JJ

43，有一种密码是由三个数组成的，这个三个数是从1－20里面任选的，问一共能产生多少个密码，数字排序相同的算一个密码
  

选项a 1160 b 7680 c 8000 d 9680 e忘记了。。。

key：????

 条件一：直线l经过点（1，-2）
    

 条件二：l的斜率为1/2

key：C????

Ok,
here comes the explanation for question 49.

 lease correct me if I am wrong. Thanks

From
condition (1), Line Y=2x-4 or Line Y=2x/3-8/3 are both examples.

Line
Y=2x-4 pass (1, -2) and (2, 0)

Line
Y=2x/3-8/3 pass (1, -2) and (4, 0)

So
condition (1) is not sufficient.

Assuming
one line pass (0, -2) and (3, 0), we can get Line Y=2x/3-2, which has slope
2/3.

Therefore,
with condition (2), the line would intersect with X at point more than x=3.

49题，简单的列个式子就行

y=ax+b

然后a已知

大小关系一目了然

比画图简单点

用代数式子解决几何问题