From definition 5.2.1:
g'(c) = limₓ→c g(x) ₋ g(c)/x₋c
Let h = x ₋ c
Then x = c₊h
substituting in the above defination we have:
g'(c) = limc₊h→c g(c₊h) ₋ g(c)/c₊h₋c
= limc₊h→c g(c₊h) ₋ g(c)/h
x → c ⇒ x ₋ c → 0 ⇒ h → 0
Hence we have,
limc₊h→c g(c₊h) ₋ g(c)/h
Therefore we have defined clearly why g′(c) in definition 5.2.1 could have been given by g′(c).
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