How to plot the graph of function $$f(x) = \sqrt{8\sin^2x+4\cos^2x-8\sin{x}\cos{x}}$$ Is it even possible ?

When I tried it the function compressed into $$f(x) = 2\sqrt{\sin^2x-2\sin{x}\cos{x}+1}$$

I can't see any way after here.


enter image description here

It's what I got using an online graph plotter. But I am expecting to plot it by hand.

  • $\begingroup$ May I suggest entering the function into wolfram to get an idea of what the graph looks like $\endgroup$
    – Déjà vu
    Jan 28, 2016 at 6:41

5 Answers 5


$\sin^2x=\dfrac{1-\cos2x}{2}$, $\sin2x=2\sin x\cos x \to\\ f(x)=2\sqrt{\dfrac{3}{2}-\dfrac{1}{2}(\sin2x+2\cos2x)}=2\sqrt{\dfrac{3}{2}-\dfrac{\sqrt{5}}{2}\sin(2x+\phi)}.$

Now you can plot by hand.


It's easier to start studying the function under the square root,


This is an homogenous trigonometric polynomial of the second degree, and we have some hope of simplifying it by the double angle formulas.



We can further simplify by the formula for linear combinations


So the graph of $g$ is a cosinusoid with period $\pi$, amplitude $\approx4.5$, phase shift $\arctan(2)$ (moved left by $\approx1.1$ radians), raised up by $6$ units. Knowing the properties of a (co)sinusoid, plotting isn't a problem (blue curve).

Now for $f(x)=\sqrt{g(x)}$, you take the square root at every point, resulting in the cosinusoid being shrunk nonlinearly (green curve). The ordinates range in $[\sqrt{6-2\sqrt5},\sqrt{6+2\sqrt5}]\approx[1.24,3.24]$.

enter image description here


Type it into wolfram alpha and you can see a plot. I guess you were not sure if the expression under the square root never negative. To check this calculate the minima of the function. This helps if you want to sketch a function by hand without a computer.

Btw $2 \sin x \cos x = \sin(2 x)$ see here. So adding a 1 to it it is never negative. So there you can see that you can plot it without calculating tyhe minima.

  • $\begingroup$ sorry for the late edit in my question. But can you suggest the way to plot it by hand? $\endgroup$
    – manshu
    Jan 28, 2016 at 6:51
  • 1
    $\begingroup$ One usually analyses the funktion by calculating the extrema (minimum and maximum) calculate the values there and draws the points. You have to realize that it is periodic (if you move $\pi$ to the right you obtain the same value.) Usually maxima and minima are enough to draw a nice enough looking line. But you may want to calculate some other point by hand and put them in. Then take a pen and connect the points so that it looks smooth. In your picture the minima and maxima are already marked with dots. The value at 0, too. That is another good value to calculate. $\endgroup$
    – user103093
    Jan 28, 2016 at 6:56
  • $\begingroup$ I guess you want to do that for an exercise. So check the exercise again if you copied it right. Because this looks unusually complicated for an exercise. Maybe there is a 8 in front of the $\cos$ and a 4 in front of the mixed term. This would make it much easier... $\endgroup$
    – user103093
    Jan 28, 2016 at 7:01
  • $\begingroup$ actually it is range that is needed to be found out in the question $\endgroup$
    – manshu
    Jan 28, 2016 at 7:08
  • $\begingroup$ and that's why I asked in the question "is it possible?" $\endgroup$
    – manshu
    Jan 28, 2016 at 7:13

First you are interesting in the $x$ axis:

  • from which $x$ is the function interesting
  • to which $x$

that gives you a rough scale, $x$ wise.

Then on the $y$ axis you want to cover at least the max and min values based on the $x$ range found above.

In this case, you see a "raw" $x$ fed to a few $\sin$ and $\cos$ without much further distortion on $x$ ; so you get a periodic function, and having the $x$ axis covering at least the $[ 0, 2\pi ]$ range should make the horizontal scale.

Without using a calculator, you see then on $y$ that roughly the function will not go to extremes, and cannot be negative. So for starters you could take $y$ in the $[0,10]$ range.

But anyway you need to draw the graph by hand, so now that the $x$ scale is roughly defined, you could use an excel like application and

  • have a column with $x$ from $0$ then below is $x$ incremented by $2\pi / 100$ etc... to have $100$ points
  • the next column is the result of the function to which the left cell is applied

You have now $100$ points $(x,y)$ to draw the graph.


I think it's possible.

By compressing, we've $f(x)=2\sqrt{(\sin x-\cos x)²+\sin² x}$.

We can see that $f$ is positive and $\pi-$periodic. So we can study it on $[0,\pi]$

As $(\sin x-\cos x)²+\sin² x>0$ and $(\sqrt{u})'=\frac{u'}{2\sqrt{u}}$,

then we've $f'(x)=\frac{((\sin x-\cos x)²+\sin² x)'}{\sqrt{(\sin x-\cos(x))²+\sin² x}}$.

But $((\sin x-\cos x)²+\sin² x)'=2(\cos x+\sin x)(\sin x-\cos x)=2(\sin² x-\cos² x)$


$(\sin² x)'=2\sin x\cos x$.

So, since $\cos(2x)=\cos²x-\sin²x$ and $\sin2x=2\sin x\cos x$,

we've $f'(x)=\frac{\sin(2x)-2\cos(2x)}{\sqrt{(\sin(x)-\cos(x))²+\sin²(x)}}$

$f'(x)=0\Leftrightarrow \sin(2x)-2\cos(2x)=\cos(2x)(\frac{\sin(2x)}{\cos(2x)}-2)=0$

In that case, $\cos(2x)=0$ or $\frac{\sin(2x)}{\cos(2x)}-2=0$

so, $x=0$ or $x={\pi \over 2}$ or $\tan(2x)=2$.

But $0$ and ${\pi \over 2}$ don't work as solutions of $\sin(2x)-2\cos(2x)=0$,

Then $2x=\arctan(2)$ so $x=\theta_0$, where $\theta_0={\arctan(2) \over2}$

We can see that when $0\leq x < \theta_0$, $\sin(2x)-2\cos(2x)<0$ and $f'(x)<0$

and when $\theta_0< x \leq \pi$, $\sin(2x)-2\cos(2x)>0$ and $f'(x)>0$

In brief, $f$ is $\pi$-periodical and has a minima on $\theta_0={\arctan(2) \over2}$(nearly 0,55 rad)


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