MHB What Is the Derivative of \( y = x \sin x \)?

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The derivative of \( y = x \sin x \) is calculated using the product rule, yielding \( \frac{dy}{dx} = \sin x + x \cos x \). This eliminates all options except for choice b. The discussion emphasizes the application of the product rule, confirming that the correct derivative involves both components of the product. The final conclusion is that the derivative is indeed \( \sin x + x \cos x \). Understanding the product rule is essential for solving similar problems.
karush
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If $y=x \sin x,$ then $\dfrac{dy}{dx}=$

$a.\quad\sin{x}+\cos{x}$
$b.\quad\sin{x}+x\cos{x}$
$c.\quad\sin{x}+\cos{x}$
$d.\quad x(\sin{x}+\cos{x})$
$e.\quad x(\sin{x}-\cos{x})$
well just by looking at it because $dx(x) = 1$
elimanates all the options besides b

$1\cdot \sin (x)+\cos (x)x$ or $\sin (x)+x\cos (x)$
otherwise the gymnastics of the product rule
$uv'+u'v$
 
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There are probably loads of proofs of this online, but I do not want to cheat. Here is my attempt: Convexity says that $$f(\lambda a + (1-\lambda)b) \leq \lambda f(a) + (1-\lambda) f(b)$$ $$f(b + \lambda(a-b)) \leq f(b) + \lambda (f(a) - f(b))$$ We know from the intermediate value theorem that there exists a ##c \in (b,a)## such that $$\frac{f(a) - f(b)}{a-b} = f'(c).$$ Hence $$f(b + \lambda(a-b)) \leq f(b) + \lambda (a - b) f'(c))$$ $$\frac{f(b + \lambda(a-b)) - f(b)}{\lambda(a-b)}...

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