In a square model of (a-b), after removing two (ab) strips from (a^2), what error may occur?

Correct answer: Not adding (b^2) back

The direct answer is D: the common error is failing to add b^2 back. Begin with the large a by a square, whose area is a^2. Removing one a by b strip subtracts ab, and removing the second strip subtracts another ab, giving a^2-2ab. However, the two strips overlap at the small b by b corner. That corner has been removed twice, although it should be absent only once. To correct this double subtraction, add b^2 back. The final result is a^2-2ab+b^2=(a-b)^2. Option A uses the plus-side model and would lead to a positive middle term. Option B counts the large square twice, which is not what the model shows. Option C misses the fact that the strips are being removed, so their contribution must be negative. Option D correctly identifies the over-subtraction of the corner. Memory cue: when two removed regions overlap, add their overlap once to repair double subtraction.