<s>[INST] Plotting $f(x) = \sin(x)+\cos(x)$ by converting it to another form Are there any trigonometric identity that can make $f(x) = \sin(x)+\cos(x)$ easier to plot? I have no idea how it becomes a sin graph shape in the end.
 [/INST] Special Case
It's not that hard. You should just use the summation formula for sines:
$$\sin (x + y) = \sin (x)\cos (y) + \cos (x)\sin (y)$$
This is how it works
$$\eqalign{
   \sin (x) + \cos (x) &= \sqrt 2 \left( {{1 \over {\sqrt 2 }}\cos (x) + {1 \over {\sqrt 2 }}\sin (x)} \right)  \cr 
   &= \sqrt 2 \left( {\sin ({\pi  \over 4})\cos (x) + \cos ({\pi  \over 4})\sin (x)} \right)  \cr 
  &= \sqrt 2 \sin (x + {\pi  \over 4}) \cr} $$
That's all you need to do for this case. If you are interested to tackle down the general case then read the sequel.

General Case 
Consider a linear combination of $\sin \alpha x$ and $\cos \alpha x$ as follows
$$y = A \cos \alpha x + B \sin \alpha x$$
where $A$ and $B$ are some real constants. Then we rewrite $y$ in this way
$$y = \sqrt{A^2+B^2} \left( \frac{A}{\sqrt{A^2+B^2}} \cos \alpha x + \frac{B}{\sqrt{A^2+B^2}} \sin \alpha x \right)$$
Now, the magic comes in! We can find a unique angle $\phi$ in the interval $[0,2\pi)$ such that
$$\begin{array}{}
\sin \phi = \dfrac{A}{\sqrt{A^2+B^2}} \\
\cos \phi = \dfrac{B}{\sqrt{A^2+B^2}}
\end{array}$$
and hence
$$y = \sqrt{A^2+B^2} \left( \sin \phi \cos \alpha x + \cos \phi \sin \alpha x \right) \\$$
Finally, using the summation formula for sines we get
$$y = \sqrt{A^2+B^2} \sin(\alpha x+\phi)$$
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