| Invalid? | Step | Justification |
| Check | a = b | Given |
| Check | ab = b^2 | Multiply both sides by b |
| Check | ab - a^2 = b^2 - a^2 | Subtract a^2 from both sides |
| Check | a(b-a) = (b + a)(b - a) | Factor both sides |
| Check | a = b + a | Divide by (b - a) |
| Check | a = a + a | Given a = b |
| Check | a = 2 * a | Simple addition |
| Check | 1 = 2 | Divide by a |
There is only one illegal step in this proof. Click on the button next to the step you think is invalid. The correctness of your choice will be displayed after a brief waiting period. The correct answer has the shortest waiting period.
Created by Andrew Freed, arf132@psu.edu on April 7, 2002.
Modified by Andrew Freed, arf132@psu.edu on April 15, 2002.
Development of this resource was sponsored by the Penn State Fund for Excellence in Learning and Teaching (FELT), project "Java-based Teaching of Mathematics in Information Sciences and Technology", supervised by Frank Ritter and David Mudgett.