请教几道jj

nicexiaoye

3. 大于700的三位整数中，有多少是奇数，要求每个数字都不为零，且每位数字都不同。答案： 91 How to get the answer?

4. is xy>z?
(1). xyz=1 (2). xyz^2>1  我觉得是C,可是很多大侠说是E,请教。

19.长方形，周长P，面积K，哪个一定对？
答案是2w^2-pw+2k=0.可是不会算

22．On Jane’s credit card account, the average daily balance for a 30-day billing cycle is the average of the daily balances at the end of each of the 30 days.At the beginning of a certain 30-day billing cycle,Jane’s credit card account had a balance of $600.Jane made a payment of $300 on the account during the billing cycle.If no other amounts were added to or subtracted from the account during the billing cycle, what was the average daily balance on Jane’s account for the billing cycle?
1) Jane’s payment was credited on the 21st day of the billing cycle.
2) The average daily balance through the 25th day of the billing cycle was $540.
The key is D,but why?

31. The three digits number is K, what is the number of tenths of K?
a. the sum of tenths of K plus 9 is 3.
b. the sum of tenths of K plus 4 is 2.

34. [(400^300+200^600)/2]^4=400^k,问K=？

suse

3.当百位数是7时，两种情况，十位数是偶数，个位数是奇数，有C（4.1）*C（4.1）=16 十位数和个位数都是奇数，有C（4.2）*P（2.2）=12 当百位数是8.也是两种，十位数是偶数，个位数是奇数，有C（3.1）*C（5.1）=15 十位数和个位数都是奇数，有C（5.2）*P（2.2）=20， 同理，百位数是9，可能性是C（4.1）*C（4.1）+C（4.2）*P（2.2）=16+12=28 所以是16+12+20+15+28=91 4.答案是C，1+2推出Z>1,xy<1， 19.设长和宽为X。Y，X+Y=P/2，X*Y=K， （X+Y)^2=X^2+2XY+Y^2=X^2+Y^2+2*K=P^2/4 X^2+Y^2=P^2/4-2*K>0,即P^2-8K>0, 而方程式要有解，要要b^2-4ac=P^2-8K>0 所以方程式一定对。 31.D，A可以求出是4.B可以求出是8