title: How to create a circle collision detection? c++
body: <p>Hi I would like to know how to create a circle collision which also acts as a sensor. For example, when the player enter the enemy's circle radius, the enemy will start chasing the player.
  Based on what I have tried, I am only using a collision box which is the <code>RECT SpriteRect</code>. But that wont be accurate enough for the circle. I was thinking about using vector by using <code>D3DXVECTOR2</code> but I am not good at transformation. Please assist me on how can I achieve this..
  <img src="https://i.stack.imgur.com/JYx1p.png" alt="enter image description here"></p>