\(\rm d\left(x-\frac{x^{3}}{3!}+\frac{x^{5}}{5!}-\frac{x^{7}}{7!} \ldots\right) \)/dx is equal to:

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  1. sin(x)
  2. ex
  3. cos(x)
  4. tan(x)

Answer (Detailed Solution Below)

Option 3 : cos(x)
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Concept:

The given function is a power series:

\[ f(x) = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots \]

This is the Taylor (Maclaurin) series expansion of \(\sin(x)\).

Calculation:

Differentiate both sides with respect to x:

\[ \frac{d}{dx} \left(x - \frac{x^3}{3!} + \frac{x^5}{5!} - \frac{x^7}{7!} + \cdots \right) = 1 - \frac{3x^2}{3!} + \frac{5x^4}{5!} - \frac{7x^6}{7!} + \cdots \]

This new series is the Maclaurin expansion of \(\cos(x)\).

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