A rectangle is made with sides (x+5) and (x+5). If it is treated as a square model, which special fact is correct?

Correct answer: The middle term is 10x

The correct answer is B. Since both sides are x+5, the rectangle is actually a square with area (x+5)². Use the square identity (p+q)²=p²+2pq+q², with p=x and q=5. This gives x²+2(x)(5)+5²=x²+10x+25. The two side strips in the visual model each have area 5x, so together they form the middle term 10x. Option A is false because the strips have equal dimensions and equal areas. Option C is false because the constant corner is 5×5=25, not 10. Option D is false because the central x-by-x region contributes x². The important distinction is that 10x comes from two rectangles, while 25 comes from the small square. Memory cue: in (x+k)², the middle term is 2kx and the constant term is k².