Since every point on the circle satisfy the equation in Q3, the equation in Q3 is the equation of circle above.
因爲圓上任意點均滿足Q3的等式，所以Q3的等式就是圖中這個圓形的方程。
4. Generally, Let C(h,k) be the centre of the circle, r be the radius of the circle. Write down the equation of the circle in terms of h,k and r. 一般來説，設C(h,k)爲圓的圓心，r爲圓的半徑。試以h, k 和r 寫下圓的方程。