To examine the intervals of increase and decrease of the function, we can observe in the illustration

What happens when the x's are smaller than the $X$ of the vertex and what happens when the x's are larger than the $X$ of the vertex?

What are the intervals of increase and decrease in this example?

We can see that the $X$ of the vertex is $-2$.

When $X>-2$ the function is increasing and, therefore, there is an interval of increase.

When $X<-2$ the function is decreasing and, therefore, there is an interval of decrease.

**What happens when there is no illustration?**

We can examine the equation of the function and determine based on the coefficient of $X^2$ whether it is a function with a minimum or maximum point.

When the coefficient is positive (happy face) - minimum

When the coefficient is negative (sad face) - maximum

Now, let's find the $X$ of the vertex according to the formula or symmetric points.

You can read more about how to find the vertex of the parabola here.

And that's it! We have all the information to determine when it is an increasing function and when it is decreasing - the vertex of the parabola is the highest or lowest point according to the concerning parabola.

All that remains for us to do is, draw a sketch to see in it the intervals of increase and decrease clearly.

**Let's see an example:**

When we see that the $X$ of the vertex is $5$

and we conclude that the parabola has a happy face, we will make a small drawing:

It can be clearly seen that the function is increasing when $X>5$ and decreasing when $X<5$ , therefore:

Interval of increase: $X>5$

Interval of decrease: $X<5$