Explanation
Mary showed that, assuming the original equation had a real solution, it
followed that the solution had to be −1. However, −1 turned out not to be a
solution. Therefore, the original equation has no real solution.
The false solution resulted from a nonreversible step. Combining equations
1 and 2 to get equation 3 is correct, but deducing equation 1 or equation 2
from equation 3 is not possible.