| ( | 15 | − | 5 | t | ) | ( | 15 | − | 5 | t | ) | + | 25 | = | 13 |
| 方程左边 = | 15 | ( | 15 | − | 5 | t | ) | − | 5 | t | ( | 15 | − | 5 | t | ) | + | 25 |
| = | 15 | × | 15 | − | 15 | × | 5 | t | − | 5 | t | ( | 15 | − | 5 | t | ) | + | 25 |
| = | 225 | − | 75 | t | − | 5 | t | ( | 15 | − | 5 | t | ) | + | 25 |
| = | 250 | − | 75 | t | − | 5 | t | ( | 15 | − | 5 | t | ) |
| = | 250 | − | 75 | t | − | 5 | t | × | 15 | + | 5 | t | × | 5 | t |
| = | 250 | − | 75 | t | − | 75 | t | + | 25 | t | t |
| = | 250 | − | 150 | t | + | 25 | t | t |
| 250 | − | 150 | t | + | 25 | t | t | = | 13 |