Answer:
![\{\frac{\pi}{4}, \frac{3\pi}{4},\frac{5\pi}{4},\frac{7\pi}{4}\}](https://tex.z-dn.net/?f=%5C%7B%5Cfrac%7B%5Cpi%7D%7B4%7D%2C%20%5Cfrac%7B3%5Cpi%7D%7B4%7D%2C%5Cfrac%7B5%5Cpi%7D%7B4%7D%2C%5Cfrac%7B7%5Cpi%7D%7B4%7D%5C%7D)
Step-by-step explanation:
We want to find the values between the interval (0, 2π) where the tangent line to the graph of y=sin(x)cos(x) is horizontal.
Since the tangent line is horizontal, this means that our derivative at those points are 0.
So, first, let's find the derivative of our function.
![y=\sin(x)\cos(x)](https://tex.z-dn.net/?f=y%3D%5Csin%28x%29%5Ccos%28x%29)
Take the derivative of both sides with respect to x:
![\frac{d}{dx}[y]=\frac{d}{dx}[\sin(x)\cos(x)]](https://tex.z-dn.net/?f=%5Cfrac%7Bd%7D%7Bdx%7D%5By%5D%3D%5Cfrac%7Bd%7D%7Bdx%7D%5B%5Csin%28x%29%5Ccos%28x%29%5D)
We need to use the product rule:
![(uv)'=u'v+uv'](https://tex.z-dn.net/?f=%28uv%29%27%3Du%27v%2Buv%27)
So, differentiate:
![y'=\frac{d}{dx}[\sin(x)]\cos(x)+\sin(x)\frac{d}{dx}[\cos(x)]](https://tex.z-dn.net/?f=y%27%3D%5Cfrac%7Bd%7D%7Bdx%7D%5B%5Csin%28x%29%5D%5Ccos%28x%29%2B%5Csin%28x%29%5Cfrac%7Bd%7D%7Bdx%7D%5B%5Ccos%28x%29%5D)
Evaluate:
![y'=(\cos(x))(\cos(x))+\sin(x)(-\sin(x))](https://tex.z-dn.net/?f=y%27%3D%28%5Ccos%28x%29%29%28%5Ccos%28x%29%29%2B%5Csin%28x%29%28-%5Csin%28x%29%29)
Simplify:
![y'=\cos^2(x)-\sin^2(x)](https://tex.z-dn.net/?f=y%27%3D%5Ccos%5E2%28x%29-%5Csin%5E2%28x%29)
Since our tangent line is horizontal, the slope is 0. So, substitute 0 for y':
![0=\cos^2(x)-\sin^2(x)](https://tex.z-dn.net/?f=0%3D%5Ccos%5E2%28x%29-%5Csin%5E2%28x%29)
Now, let's solve for x. First, we can use the difference of two squares to obtain:
![0=(\cos(x)-\sin(x))(\cos(x)+\sin(x))](https://tex.z-dn.net/?f=0%3D%28%5Ccos%28x%29-%5Csin%28x%29%29%28%5Ccos%28x%29%2B%5Csin%28x%29%29)
Zero Product Property:
![0=\cos(x)-\sin(x)\text{ or } 0=\cos(x)+\sin(x)](https://tex.z-dn.net/?f=0%3D%5Ccos%28x%29-%5Csin%28x%29%5Ctext%7B%20or%20%7D%200%3D%5Ccos%28x%29%2B%5Csin%28x%29)
Solve for each case.
Case 1:
![0=\cos(x)-\sin(x)](https://tex.z-dn.net/?f=0%3D%5Ccos%28x%29-%5Csin%28x%29)
Add sin(x) to both sides:
![\cos(x)=\sin(x)](https://tex.z-dn.net/?f=%5Ccos%28x%29%3D%5Csin%28x%29)
To solve this, we can use the unit circle.
Recall at what points cosine equals sine.
This only happens twice: at π/4 (45°) and at 5π/4 (225°).
At both of these points, both cosine and sine equals √2/2 and -√2/2.
And between the intervals 0 and 2π, these are the only two times that happens.
Case II:
We have:
![0=\cos(x)+\sin(x)](https://tex.z-dn.net/?f=0%3D%5Ccos%28x%29%2B%5Csin%28x%29)
Subtract sine from both sides:
![\cos(x)=-\sin(x)](https://tex.z-dn.net/?f=%5Ccos%28x%29%3D-%5Csin%28x%29)
Again, we can use the unit circle. Recall when cosine is the opposite of sine.
Like the previous one, this also happens at the 45°. However, this times, it happens at 3π/4 and 7π/4.
At 3π/4, cosine is -√2/2, and sine is √2/2. If we divide by a negative, we will see that cos(x)=-sin(x).
At 7π/4, cosine is √2/2, and sine is -√2/2, thus making our equation true.
Therefore, our solution set is:
![\{\frac{\pi}{4}, \frac{3\pi}{4},\frac{5\pi}{4},\frac{7\pi}{4}\}](https://tex.z-dn.net/?f=%5C%7B%5Cfrac%7B%5Cpi%7D%7B4%7D%2C%20%5Cfrac%7B3%5Cpi%7D%7B4%7D%2C%5Cfrac%7B5%5Cpi%7D%7B4%7D%2C%5Cfrac%7B7%5Cpi%7D%7B4%7D%5C%7D)
And we're done!
Edit: Small Mistake :)