1.③, which is representative.

2. Take 5 middle schools in each city, and take 5 students from each class in each middle school to measure their blood pressure.

3.1.52m

4. 12 15

5.( 1) mode 2, median1;

(2) Mode 9, median 9 ..

6.( 1) median: 5 modulo 4, with an average of 5.6.

(2) Omission

7.0.02 square kilograms

8.b, the average grade is good and the grade is stable.

9.( 1) Average A: 2.5 million yuan/month B: 2.3 million yuan/month

(2) A0.012b1.1616 reflects the degree of data dispersion.

10.( 1) The average value before the gate is 127m, the variance is 406m, and the standard deviation is 20.1m..

After the close, the average value is 148m, the variance is 143m, and the standard deviation is12.0m. ..

(2) With the increase of average altitude, the variance of average altitude becomes smaller.