Let there be two real numbers a, b such that |a|≥|b|. Let there be a third real number, c, such that c=(a+b)/2.
Suppose that |c|>|a|:
For the case a=b=0, c=a=b=(a+b)/2 = 0, thus the statement |c| > max(|a|,|b|) is incorrect in this case.
For the case |a|> 0 and |b|< 0, as well as the case |a| > 0 and |b|<0 the contradiction is apparent. As |a+b| ≤ |a|. Following this, |(a+b)/2| > |a| is impossible. Proving that the average, c is bounded within the range (b,a) or the range (a,b).
For the a≥0 and b≥0, as well as the case a≤0 and b≤0, let us rewrite the average function.
Average = (a+a-x))/2, where x is the difference between a and b. Since we are proving that the statement |c|>|a| is never true, let us take the norm of the average function
|Average| = (|a|+|a-x|))/2
Clearly, this function is maximized when x is equal to zero. Which gives
|Average| = |2a|/2 = |a|.
Thus, max(|c|) = |a|
This proves that the average two real numbers a,b is bounded between the closed interval [a,b]