| ( | 2020 | − | a | ) | ( | 2019 | − | a | ) | = | 2020 |
| Left side of the equation = | 2020 | ( | 2019 | − | a | ) | − | a | ( | 2019 | − | a | ) |
| = | 2020 | × | 2019 | − | 2020 | a | − | a | ( | 2019 | − | a | ) |
| = | 4078380 | − | 2020 | a | − | a | ( | 2019 | − | a | ) |
| = | 4078380 | − | 2020 | a | − | a | × | 2019 | + | a | a |
| = | 4078380 | − | 4039 | a | + | a | a |
| 4078380 | − | 4039 | a | + | a | a | = | 2020 |