( | 15 | − | 3 | x | ) | ( | 20 | − | 4 | x | ) | = | 108 |
方程左边 = | 15 | ( | 20 | − | 4 | x | ) | − | 3 | x | ( | 20 | − | 4 | x | ) |
= | 15 | × | 20 | − | 15 | × | 4 | x | − | 3 | x | ( | 20 | − | 4 | x | ) |
= | 300 | − | 60 | x | − | 3 | x | ( | 20 | − | 4 | x | ) |
= | 300 | − | 60 | x | − | 3 | x | × | 20 | + | 3 | x | × | 4 | x |
= | 300 | − | 60 | x | − | 60 | x | + | 12 | x | x |
= | 300 | − | 120 | x | + | 12 | x | x |
300 | − | 120 | x | + | 12 | x | x | = | 108 |