\( \DeclareMathOperator{cosec}{cosec} \)
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Differentiation from first principles is a method used to find the derivative of a function by computing the limit of the difference quotient as the change in the input variable approaches zero. In other words, it involves finding the gradient of a curve at a point by zooming in very closely and calculating the slope of the tangent line.
The key formula for differentiation from first principles is:
$$f'(x) = \lim_{h\to0} \frac{f(x+h) - f(x)}{h}$$where \(f(x)\) is the function to be differentiated.
Here is an example of how to use differentiation from first principles to find the derivative of the function \(f(x) = x^2\):
$$\begin{aligned} f'(x) &= \lim_{h\to0} \frac{f(x+h) - f(x)}{h} \\ &= \lim_{h\to0} \frac{(x+h)^2 - x^2}{h} \\ &= \lim_{h\to0} \frac{x^2 + 2xh + h^2 - x^2}{h} \\ &= \lim_{h\to0} \frac{2xh + h^2}{h} \\ &= \lim_{h\to0} (2x + h) \\ &= 2x \end{aligned} $$Therefore, the derivative of \(f(x) = x^2\) is \(f'(x) = 2x\).
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