Revision on equation of Straight line
Consider the straight line passing through points A and B. C is a point on the straight line. 考慮穿過A，B兩點的直綫。C是綫上一點。
1. Let C be (x,y).
(a) From the graph, Which of the following conditions does C satisfy? 由圖中可得，點C滿足以下哪個條件?
(b) From (a), determine which of the following equations does C satisfy? 由(a)，點C滿足以下哪條等式?
Since every points on the straight line satisfy the equation above, the equation above is the equation of the straight line.
因爲直綫上的任意點均滿足上述式，因此上述式就是這條直綫的方程。
2. 透過化簡上述式，可得以下直綫方程。
Conclusion
Therefore, to find the equation of any shape, we just need to find an equation that every points on the shape satisfy.所以，求任意圖形的方程，我們衹需要找出圖形上任意點都滿足的一條等式。