作图法：看见交点
作图法把每个方程画成一条直线，再读出交点。它能建立最清晰的直觉，但精度只取决于你画得多准——交点在 (2.5, 1.7) 时很难从草图上读出。把每条直线改写成斜截式 y = mx + b，便于快速作图。
代入法：先解出，再替换
当某个变量已经独立、或容易被孤立出来时，代入法最为出色。你先用孤立变量把一个变量表示出来，再代入另一个方程，把两个未知数压缩成一个。
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
消元法把整个方程相加或相减，使某个变量抵消。先把每个方程乘以适当倍数，让某个变量的系数互为相反数，然后相加。这种方法最适合推广到更大的方程组。
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 ✓