After reading this question, I will write Q 1/2. I will write the answer without hesitation, and I will never miss the exam. You can write the answer boldly, because the information given in the test paper is definitely not useless. ) But I usually do it again. Let an = A 1Q N- 1 and tn = A 1NQ N (N- 1)/2, then TN = A 1Q- 1/2Q N.