I'm reading this text:

[![enter image description here](https://i.stack.imgur.com/jM21R.png)](https://i.stack.imgur.com/jM21R.png)

$$\int\_0^1 \sqrt{1 - x^2} \cdot dx$$

Why isn't the above integral the right half of a circle? Why did they show the equation of $y^2$? Was the point of that just to show that it's a circle? But that just confuses people because they don't really explain why it's just 1 quarter of a circle. I guess y cannot be negative? But $y$ can be negative in the $y^2$ equation.