2400 | − | ( | 40 | − | 2 | a | ) | ( | 60 | − | 2 | a | ) | = | 900 |
方程左边 = | 2400 | − | 40 | ( | 60 | − | 2 | a | ) | + | 2 | a | ( | 60 | − | 2 | a | ) |
= | 2400 | − | 40 | × | 60 | + | 40 | × | 2 | a | + | 2 | a | ( | 60 | − | 2 | a | ) |
= | 2400 | − | 2400 | + | 80 | a | + | 2 | a | ( | 60 | − | 2 | a | ) |
= | 0 | + | 80 | a | + | 2 | a | ( | 60 | − | 2 | a | ) |
= | 80 | a | + | 2 | a | × | 60 | − | 2 | a | × | 2 | a |
= | 80 | a | + | 120 | a | − | 4 | a | a |
= | 200 | a | − | 4 | a | a |
200 | a | − | 4 | a | a | = | 900 |