| ( | 10 | + | x | ) | ( | 40 | − | 1 2 | x | ) | = | 900 |
| Left side of the equation = | 10 | ( | 40 | − | 1 2 | x | ) | + | x | ( | 40 | − | 1 2 | x | ) |
| = | 10 | × | 40 | − | 10 | × | 1 2 | x | + | x | ( | 40 | − | 1 2 | x | ) |
| = | 400 | − | 5 | x | + | x | ( | 40 | − | 1 2 | x | ) |
| = | 400 | − | 5 | x | + | x | × | 40 | − | x | × | 1 2 | x |
| = | 400 | + | 35 | x | − | x | × | 1 2 | x |
| 400 | + | 35 | x | − | x | × | 1 2 | x | = | 900 |