ข้อสอบคณิต O-NET - 2559

$$\begin{gathered} \mathrm{จากโจทย์} \\ \mathrm{จาก} a_4 = 26 \mathrm{และ} a_{n+1} = a_n+n \\ \mathrm{แทน} n=3 \mathrm{จะได้} a_4 = a_3+3 \\ 26 = a_3+3 \\ a_3 = 23 \dots \overline{)1} \end{gathered}$$

$$\begin{gathered} \mathrm{จาก} a_3 = 23 \mathrm{และ} a_{n+1} = a_n+n \\ \mathrm{แทน} n=2 \mathrm{จะได้} a_3 = a_2+2 \\ 23 = a_2+2 \\ a_2 = 21 \dots \overline{)2} \end{gathered}$$

$$\begin{gathered} \mathrm{จาก} a_2 = 21 \mathrm{และ} a_{n+1} = a_n+n \\ \mathrm{แทน} n=1 \mathrm{จะได้} a_2 = a_1+1 \\ 21 = a_1+1 \\ a_1 = 20 \dots \overline{)3} \end{gathered}$$

$$\begin{gathered} \mathrm{จาก} \overline{)1} , \overline{)2} , \overline{)3} \mathrm{จะได้} : a_1+a_2+a_3 \\ = 20+21+23 \\ = 64 \end{gathered}$$