I think such a reaction is desirable because the student knows that even though he did not do something well, he showed some knowledge. Later we will also practice examples of "opposite" (we modify the multiplied expression to the form "brackets"). This procedure makes it more difficult for students, they probably do not encounter it so often and they are not so confident in solving it. In the last third of the lesson, students are asked to pull out a worksheet.
This procedure makes it more difficult for students, they probably do not encounter it so often and they are not so confident in solving it.