文字数制限かかるのでインデント調整無しで
Point location = new Point(0, 0);
Size size = new Size(100, 100);
private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
e.Graphics.FillEllipse(Brushes.Red, location.X, location.Y, size.Width, size.Height);
}

private void pictureBox1_Click(object sender, EventArgs e)
{
int r = 50;
Point p = PointToClient(new Point(
Cursor.Position.X,
Cursor.Position.Y));
int x = p.X;
int y = p.Y;
int a = x - (location.X + size.Width/2);
int b = y - (location.Y + size.Height/2);
int cc = a * a + b * b;
if (cc <= r * r)
{
MessageBox.Show("true");
}
else
{
MessageBox.Show("false");
}
}