https://www.studyinjapan.go.jp/en/planning/scholarships/mext-scholarships/examination.html Answered by https://math-gpt.org/ (6) Sequence The sequence is 1 , 4 , 10 , 19 , 31 , … 1,\ 4,\ 10,\ 19,\ 31,\ldots 1 , 4 , 10 , 19 , 31 , … Find the differences: Term 1 4 10 19 31 First difference 3 6 9 12 Second difference 3 3 3 \begin{array}{c|ccccc} \text{Term} & 1 & 4 & 10 & 19 & 31\\ \hline \text{First difference} & & 3 & 6 & 9 & 12\\ \text{Second difference} & & & 3 & 3 & 3 \end{array} Term First difference Second difference 1 4 3 10 6 3 19 9 3 31 12 3 Since the second difference is constant, the sequence is quadratic. The first difference before the first term is 0 0 0 , so the n n n -th term can be found by adding the differences: a n = 1 + 3 ( 1 + 2 + ⋯ + ( n − 1 ) ) . a_n=1+3(1+2+\cdots +(n-1)). a n = 1 + 3 ( 1 + 2 + ⋯ + ( n − 1 )) . Using 1 + 2 + ⋯ + ( n −...