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Worked Examples · Example 9

Q.Find the derivative of f(x)=10xf(x) = 10x.

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The derivative measures the rate of change of a function; for f(x)=10xf(x) = 10x, the output changes by 1010 units for every unit change in xx, so f′(x)=10f'(x) = 10.

Understanding the derivative at a point

The derivative of a function at any point tells us how fast the function is changing there. For a linear function like f(x)=10xf(x) = 10x, the graph is a straight line through the origin with slope 1010. This slope is constant everywhere—the function climbs by 1010 units vertically for every 11 unit we move horizontally. That constant rate of change is the derivative.

We can verify this using the limit definition of the derivative:

f′(x)=lim⁡h→0f(x+h)−f(x)hf'(x) = \lim_{h \to 0} \frac{f(x+h) - f(x)}{h}

This formula captures the idea of "rise over run" as the run shrinks to zero.

Step-by-step calculation

  1. Substitute into the definition. We have f(x)=10xf(x) = 10x, so f(x+h)=10(x+h)=10x+10hf(x+h) = 10(x+h) = 10x + 10h. The difference quotient becomes:

f(x+h)−f(x)h=(10x+10h)−10xh=10hh\frac{f(x+h) - f(x)}{h} = \frac{(10x + 10h) - 10x}{h} = \frac{10h}{h}

  1. Simplify before taking the limit. As long as h≠0h \neq 0 (which is fine, since we're taking a limit as hh approaches zero, not equals it), we can cancel:

10hh=10\frac{10h}{h} = 10

  1. Evaluate the limit. …

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