why range is nt a measure of central tendency ???

Answer  :
We know " Range and measures of central tendency (mean, median and mode) are values that summarise a set of data.  They are useful when analysing data. " 
And

A measure of central tendency is a single value that attempts to describe a set of data by identifying the central position within that set of data. As such, measures of central tendency are sometimes called measures of central location. They are also classed as summary statistics. The mean (often called the average) is most likely the measure of central tendency that you are most familiar with, but there are others, such as the median and the mode.

Let we have data As :
Daily highest temperature for different years

2001	2002	2005	2008	2010	2013
13	50	40	47	50	59

So Range  : the difference between the greatest and the least values in a data set
Range  = 59 - 13   = 46

Mean : the sum of a set of data divided by the number of data

Mean  = $\frac{13 + 50 +\hspace{0.17em}40 + 47 + 50 + 59}{6}$$\frac{13 +\hspace{0.17em}50 + 40 + 47 + 50 +59}{6}$

= 43.16

Median : the middle value of a data set
To find the median, place all the data in numerical order, then find the middle number.  If there are two middle numbers, find the mean (or average) of the two middle numbers.
13 , 40 , 47 , 50 , 50 , 59
SO median 47 + 50  = 97
$\frac{97}{2}$ = 48.5

Mode : the most common value in a data set

Here that is 50 ( As it come more than any other )

So,

Range is different from mean mode and median , that why we separate it from measure of central tendency .