Limit Definition of Derivative
Key Questions
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Limit Definition of
#f'(a)# #f'(a)=lim_{h to 0}{f(a+h)-f(a)}/h# or
#f'(a)=lim_{x to a}{f(x)-f(a)}/{x-a}# -
#f(x)=c# is a constant function, so its value stays the same regardless of the x-value. In particular,#f(x+h)=c# .By the definition of the derivative,
#f'(x)=lim_{h to 0}{f(x+h)-f(x)}/h# #=lim_{h to 0}{c-c}/{h}# #=lim_{h to 0}0# #=0# -
Remember that the limit definition of the derivative goes like this:
#f'(x)=lim_{h rightarrow0}{f(x+h)-f(x)}/{h}# .
So, for the posted function, we have
#f'(x)=lim_{hrightarrow0}{m(x+h)+b-[mx+b]}/{h}#
By multiplying out the numerator,
#=lim_{hrightarrow0}{mx+mh+b-mx-b}/{h}#
By cancelling out#mx# 's and#b# 's,
#=lim_{hrightarrow0}{mh}/{h}#
By cancellng out#h# 's,
#=lim_{hrightarrow0}m=m#
Hence,#f'(x)=m# .The answer above makes sense since the derivative tells us about the slope of the tangent line to the graph of
#f# , and the slope of the linear function (its graph is a line) is#m# . -
Yes, there is a difference since the first limit is defined at
#x=0# , but the second one is not.
I hope that this was helpful.
Questions
Derivatives
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Tangent Line to a Curve
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Normal Line to a Tangent
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Slope of a Curve at a Point
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Average Velocity
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Instantaneous Velocity
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Limit Definition of Derivative
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First Principles Example 1: x²
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First Principles Example 2: x³
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First Principles Example 3: square root of x
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Standard Notation and Terminology
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Differentiable vs. Non-differentiable Functions
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Rate of Change of a Function
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Average Rate of Change Over an Interval
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Instantaneous Rate of Change at a Point