When h = 0 , [f(x + h) - f(x)]/h = 2x +2 for the given expression.
The missing expression is f(x) = x² + 2x.
What is formula of difference quotient ?
f(x+h)-f(x)/h is called the formula of difference quotient.
When h---> 0 ,
the expression f'(x) = (f(x-h) - f(x)) / h is the equation for tangent to the line
Here the the expression is
f(x) = x² + 2x
Determining f(x + h) by substituting x = x + h on both sides of the given f(x).
Then f(x + h) = (x + h)² + 2(x + h)
= x² + 2xh + h² + 2x + 2h
the difference f(x + h) - f(x).
f(x + h) - f(x) = [x² + 2xh + h² + 2x + 2h] - [x² + 2x]
= 2xh + h² + 2h
Divide the difference from h as in the formula of difference coefficient
[f(x + h) - f(x)]/h = (2xh + h² + 2h) / h
= 2x + h + 2
So, when h = 0 , [f(x + h) - f(x)]/h = 2x +2
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