作圖法：看見交點
作圖法把每個方程畫成一條直線，再讀出交點。它能建立最清晰的直覺，但精度只取決於你畫得多準——交點在 (2.5, 1.7) 時很難從草圖上讀出。把每條直線改寫成斜截式 y = mx + b，便於快速作圖。
代入法：先解出，再替換
當某個變量已經獨立、或容易被孤立出來時，代入法最為出色。你先用孤立變量把一個變量表示出來，再代入另一個方程，把兩個未知數壓縮成一個。
Solve: y = 2x - 1 (already solved for y)
3x + y = 9
Substitute y = 2x - 1 into the second equation:
3x + (2x - 1) = 9
5x - 1 = 9
5x = 10
x = 2
Back into y = 2x - 1:
y = 2(2) - 1 = 3
Solution: (2, 3). Check: 3(2) + 3 = 9 ✓消元法：相加抵消
消元法把整個方程相加或相減，使某個變量抵消。先把每個方程乘以適當倍數，讓某個變量的係數互為相反數，然後相加。這種方法最適合推廣到更大的方程組。
Solve: 2x + 3y = 12
4x - 3y = 6
The +3y and -3y are already opposites. Add the equations:
(2x + 4x) + (3y - 3y) = 12 + 6
6x = 18
x = 3
Back-substitute x = 3 into 2x + 3y = 12:
6 + 3y = 12 -> 3y = 6 -> y = 2
Solution: (3, 2). Check in eq 2: 4(3) - 3(2) = 6 ✓