( | 140 | − | 100 | − | a | ) | ( | 300 | − | 2 | a | ) | = | 1200 |
方程左边 = | 140 | ( | 300 | − | 2 | a | ) | − | 100 | ( | 300 | − | 2 | a | ) | − | a | ( | 300 | − | 2 | a | ) |
= | 140 | × | 300 | − | 140 | × | 2 | a | − | 100 | ( | 300 | − | 2 | a | ) | − | a | ( | 300 | − | 2 | a | ) |
= | 42000 | − | 280 | a | − | 100 | ( | 300 | − | 2 | a | ) | − | a | ( | 300 | − | 2 | a | ) |
= | 42000 | − | 280 | a | − | 100 | × | 300 | + | 100 | × | 2 | a | − | a | ( | 300 | − | 2 | a | ) |
= | 42000 | − | 280 | a | − | 30000 | + | 200 | a | − | a | ( | 300 | − | 2 | a | ) |
= | 12000 | − | 80 | a | − | a | ( | 300 | − | 2 | a | ) |
= | 12000 | − | 80 | a | − | a | × | 300 | + | a | × | 2 | a |
= | 12000 | − | 380 | a | + | a | × | 2 | a |
12000 | − | 380 | a | + | a | × | 2 | a | = | 1200 |