1
$\begingroup$

Again a simple problem that I can't seem to get the derivative of

I have $\frac{x}{2} + \frac{1}{4}\sin(2x)$

I am getting $\frac{x^2}{4} + \frac{4\sin(2x)}{16}$

This is all very wrong, and I do not know why.

  • 0
    @5tom I still do not see how that equals $cos^2 x$2012-04-21

6 Answers 6

1

$y = \dfrac{1}{2} x + \dfrac {1}{4} \sin(2x)$

$y' = \dfrac{1}{2} + \dfrac{1}{4}\cos(2x)*2$

$y' = \dfrac{1}{2} + \dfrac{2}{4}\cos(2x)$

$y' = \dfrac{1}{2} + \dfrac{1}{2}\cos(2x)$

$y' = \dfrac{1}{2} \big(1 + \cos (2x)\big) $

Which is equivalent to $\cos^2x$.

5

You want the derivative of a sum of functions, so find the derivative of each term in the sum and add.

For the term ${x\over2}$, note that it can be written as ${1\over2} x$. Now use the fact that you can factor constants out of derivatives: $\color{maroon}{ {d\over dx }{x\over 2 } } = {d\over dx }\Bigr({1\over 2 } x \Bigl) ={1\over 2 } {d\over dx } x ={1\over 2}\cdot 1=\color{maroon}{{1\over2}}. $

For the other term, you'll need to use the chain rule and the fact that the derivative of $\sin(x)$ is $\cos(x)$: $\color{darkgreen}{ {d\over dx }\Bigr({1\over 4} \sin(2x) \Bigl) } ={1\over 4} {d\over dx } \sin(2x) = {1\over 4}\cos(2x) \cdot (2x)' = {1\over 4}\cos(2x) \cdot2=\color{darkgreen}{ {1\over 2}\cos(2x)}. $

Combining the above results: $ {d\over dx}\Bigl( {x\over2}+{1\over4}\sin(2x)\Bigr)= \color{maroon}{{d\over dx} {x\over2}}+\color{darkgreen}{ {d\over dx}\Bigr( {1\over4}\sin(2x)\Bigr) } =\color{maroon}{{1\over2}}+\color{darkgreen}{{1\over 2}\cos(2x) }={1\over2}\bigl(1+\cos(2x)\bigr). $

3

You deal with the sum of functions, $f(x) = \frac{x}{2}$ and $g(x)= \frac{1}{4} \sin(2 x)$. So you would use linearity of the derivative: $ \frac{d}{d x} \left( f(x) + g(x) \right) = \frac{d f(x)}{d x} + \frac{d g(x)}{d x} $ To evaluate these derivatives, you would use $\frac{d}{d x}\left( c f(x) \right) = c \frac{d f(x)}{d x}$, for a constant $c$. Thus $ \frac{d}{d x} \left( \frac{x}{2} + \frac{1}{4} \sin(2 x) \right) = \frac{1}{2} \frac{d x}{d x} + \frac{1}{4} \frac{d \sin(2 x)}{d x} $ To evaluate derivative of the sine function, you would need a chain rule: $ \frac{d}{d x} y(h(x)) = y^\prime(h(x)) h^\prime(x) $ where $y(x) = \sin(x)$ and $h(x) = 2x$. Now finish it off using table of derivatives.

  • 0
    @Jordan Sasha gave a good help. Try and read your notes carefully to understand what theorems and properties of the derivative you need to solve the problem.2012-04-21
2

Jordan, The derivative of your function is $\frac{1}{2} + \frac{\cos 2x}{2}$. Now note that $\cos 2x = \cos^2 x -\sin ^2 = \cos^2 x -1 -\cos^2 x =2\cos^2x -1$. Rearranging, you get $\cos^2 x =\frac{\cos 2x}{2} + \frac{1}{2}.$


$ \begin{align*} \cos 2x = \cos(x+x) & =\cos x \cos x -\sin x \sin x \\ & = \cos^2x -\sin^2x\\ & = \cos^2x -(1-\cos^2x)\qquad\text{because}~\cos^2x + \sin^2 x =1.\\ & = \cos^2x-1+ \cos^2x\\ & = 2\cos^2x-1 \end{align*} $

So, you have $\cos2x = 2\cos^2x -1$, which is the same as $\cos 2x + 1 = 2\cos^2x$. Divide both sides by 2 to get what you want.

  • 0
    Where cos2x/2 + 1/2 is equal to cos^2 x2012-04-21
1

If your trying to prove integrals you should make your question a little better, because the current question makes it appear that you do not know how to take a derivative.

1

If $f(x) = \frac{x}{2} + \frac{1}{4}\sin(2x)$, then $f'(x) =$ $\frac{1}{2} + \frac{cos(2x)}{2}$. If you want $\int{f(x)}$, then we have $\frac{x^2}{4} - \frac{cos(2x)}{8}+C$. Here, we differentiate/integrate $\frac{x}{2}$ and $\frac{1}{4}\sin(2x)$ separately. Do you have an initial condition?

  • 0
    That is not the derivative of your function.2012-04-21