| ( | x | − | 40 | ) | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) | = | 400 |
| 方程左边 = | x | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) | − | 40 | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) |
| = | x | × | 20 | + | x | × | 2 | ( | 60 | − | x | ) | − | 40 | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) |
| = | 20 | x | + | x | × | 2 | × | 60 | − | x | × | 2 | x | − | 40 | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) |
| = | 20 | x | + | x | × | 120 | − | x | × | 2 | x | − | 40 | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) |
| = | 140 | x | − | x | × | 2 | x | − | 40 | ( | 20 | + | 2 | ( | 60 | − | x | ) | ) |
| = | 140 | x | − | x | × | 2 | x | − | 40 | × | 20 | − | 40 | × | 2 | ( | 60 | − | x | ) |
| = | 140 | x | − | x | × | 2 | x | − | 800 | − | 80 | ( | 60 | − | x | ) |
| = | 140 | x | − | x | × | 2 | x | − | 800 | − | 80 | × | 60 | + | 80 | x |
| = | 140 | x | − | x | × | 2 | x | − | 800 | − | 4800 | + | 80 | x |
| = | 220 | x | − | x | × | 2 | x | − | 5600 |
| 220 | x | − | x | × | 2 | x | − | 5600 | = | 400 |