If the average of data 8, 9, 7, 8, x, 4 is 7, then the variance of this set of data is______ ．

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• 1. Given a group of data 9,10,11, x, y, the average is 10, the variance is 2, find this group of data
• 2. A group of data is x y 9 10 11, the average of this group of data is 10, then the minimum variance is 0
• 3. It is known that the average and variance of the first 20 data are 60 and 20 respectively, and the average and variance of the last 20 data are 80 and 40 respectively
• 4. If the average of sample data 3, 6, a, 4 and 2 is 5, then the variance of this sample is () A. 8B. 5C. 22D. 3
• 5. If the average of a set of data 2, 4, x, 6, 8 is 6, then the standard deviation of this set of data is () A. 10B. 8C. 6D. 22
• 6. The variance of sample data 10 10.19.7 9.9 10.3 is the mean of data 1-20-11, is the variance s #; yes
• 7. Among the 10 known data, the average and variance of the first nine data are 1 and 7, respectively The 10th data is equal to 11, so what is the variance of the 10th data?
• 8. If the average of a group of sample data 2, 3, 7, 8, a is 5, then the variance S2 of the group of data is 0=______ ．
• 9. Given that the average of data a, B and C is D and the variance is e, what is the average and variance of a group of new data 3a-2, 3b-2 and 3c-2?
• 10. The known elements are integer sequence {1}, {2,3}, {4,5,6}, {7,8,9,10} Starting from the second sequence, each number set has one more number than the previous number set, and the minimum number of each number set is 1 larger than the maximum number of the previous number set. Find the sum SN of all numbers in the nth number set
• 11. The time Xiao Ming spent on his way to school five times is x y 10 11 9. Given that the average of this group of data is 10 minutes and the variance is 2, what is the absolute value of X-Y? A (1) B (2) C (3) d (4), please write down the whole process,
• 12. The time (unit: minutes) that someone spends on the way to work five times are x, y, 10, 11 and 9 respectively. Given that the average of this set of data is 10 and the variance is 2, then the value of | X-Y | is () A. 1B. 2C. 3D. 4
• 13. The time (unit: minutes) that someone spends on the way to work five times are x, y, 10, 11 and 9 respectively. Given that the average of this set of data is 10 and the variance is 2, then the value of | X-Y | is () A. 1B. 2C. 3D. 4
• 14. The time (minutes) taken by a person on his way to work five times are x, y, 14, 12 and 13 respectively. Given that the average of this set of data is 12 and the variance is 2, then the value of | X-Y | is 0
• 15. If the average number of time spent on the way to work is 10 and the variance is 2, then the value of X-Y is 0 The process should be detailed`
• 16. In another shooting test, the average number of rings of a, B, C and D is the same, but the variance is 8.7, 6.5, 9.1 and 7.7 respectively Q: what is the most stable shooting result among the four A. A b b c c c d
• 17. In a shooting competition, the result of a country's 10 times is 92 rings, and that of B country's 7 times is 9,10,9,10,10,9,8 respectively. If B country's athlete wants to surpass that of a country, how many rings should he hit in the last 3 times?
• 18. A certain organic compound A is composed of C, h and O. under certain conditions, the conversion relationship among a, B, C, D and E is as follows: it is known that the vapor density of C is 22 times higher than that of hydrogen under the same conditions, and silver mirror reaction can occur. (1) the name of the functional group in substance D is______ The structure formula of substance e is______ (2) write out the chemical equation to realize the following transformation: ① a → B______ ，②A+O 2 →C______ ③ The reaction equation of C with silver ammonia solution______ ．
• 19. With the four numbers of 0, 5, 8 and 7, we can form () different four digit numbers? (1) 10 (2) 18 (3) 11 (4) 9
• 20. 0-1+2-3+4-5+6-7+8-9+10-.-99+100 0.1+0.3+0.5+0.7+0.9+0.11+0.13+.+0.97+0.99