The r-chart
a. is used to indicate gains or losses in uniformity.
b. generally uses control limits set at plus or minus 2 standard deviations of the distribution, rather than plus or minus 3 which is commonly used on the x-bar chart.
c. control limits are computed using sample standard deviations.
d. is used to measure changes in the central tendency.

A.

Step-by-step explanation:

An R-Chart is a control chart used to indicate the changes in a variable usually  with processes that have subgroup size of 2 or higher. Based on the answers provided within the question, it can be said that the R-chart is used to indicate gains or losses in uniformity, which would be the changes in the variable. This can be demonstrated in the R-chart below.

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arithmetic sequence

step-by-step explanation:

given that a sequence has terms as

19, 15, 11,

this is represented on a graph

the pattern indicates that the sequence is an arithmetic sequence with i term

a=19 and common difference d =-4

any general term

thus the sequence is reduced by 4 for each successive term starting from 1 and nth term is

24-5n.

first negative term would start after 5th term

5th term =24-25< 0

from 5th term onwards the sequence would be negative.