| x | x | + | ( | 20 | − | x | ) | ( | 20 | − | x | ) | = | 25 2 |
| Left side of the equation = | x | x | + | 20 | ( | 20 | − | x | ) | − | x | ( | 20 | − | x | ) |
| = | x | x | + | 20 | × | 20 | − | 20 | x | − | x | ( | 20 | − | x | ) |
| = | x | x | + | 400 | − | 20 | x | − | x | ( | 20 | − | x | ) |
| = | x | x | + | 400 | − | 20 | x | − | x | × | 20 | + | x | x |
| = | x | x | + | 400 | − | 40 | x | + | x | x |
| x | x | + | 400 | − | 40 | x | + | x | x | = | 25 2 |