# Point to lie on a circle. Help!

• 08-05-2011
plus.c.plus
Point to lie on a circle. Help!
Please, would you help me with this program:

Code:

``` #include <iostream>  #include <cmath>  using namespace std;  void main()  {  cout << "x = ";  double x;  cin >> x;  cout << "y = ";  double y;  cin >> y;  bool c1 = ((((x + 1)*(x + 1) + (y - 1)*(y - 1)) <= 1) &&  (y <= (1/3*x + 1)) &&  (y >= (x + 1)) &&  (y >= 0));  if (c1)  cout << "true" << endl;  else  cout << "false" << endl;  }```

The point don`t have to lie in (-2, 0.5), (-2, 1) and (0,1). :S
• 08-05-2011
AndrewHunter
Sure:

1. It's int main(). Read why void main() is wrong
2. Learn how to indent your code.
3. Learn how to ask questions.
• 08-06-2011
Salem
On a circle?
In a circle?

We need more than some random uncommented code, and an unexplained picture.
• 08-07-2011
quzah
Shouldn't you have more than just one set of numbers? Like D is the diameter of the circle, and X,Y is the coordinate of the center of the circle, and pX,pY is the coordinates of the point you want to check? You have X and Y, but what are you comparing X and Y to? You can't find out if something is relative to a coordinate, if you only have one of the two pairs of coordinates.

"Is this point on a line?"
"Maybe."

Quzah.
• 08-07-2011
VirtualAce
Quzah is right. You cannot solve this problem without knowing the center of the circle and the radius or diameter of the circle. Since a circle is the set of all points equidistant from a center point the check to see if px,py lies on the circle is simple if you know those two pieces of information about the circle.
• 08-07-2011
iMalc
From the diagram, we know the centre of the circle. I think what we don't know is what the original poster is actually asking.
• 08-07-2011
plus.c.plus
Thank you! :)
• 08-07-2011
VirtualAce
For what? You have not explained what you want.
• 08-08-2011
quzah
I still don't know what that shaded area is supposed to be. The area of triangle 0/-1,-1/0,-3/0 that happens to fall in the circle? That doesn't really do anything for us as far as finding "a point to lie on a circle".

Quzah.