The teacher immediately realized how the mistake had arisen, so she didn't have to react to it in a purely negative way, but in the style of "it would be if there was a plus". 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").
I think such a reaction is desirable because the student knows that even though he did not do something well, he showed some knowledge.