ข้อสอบคณิต O-NET - 2551

$$\begin{gathered} \mathrm{จากสูตร} \mathrm{พจน์ทั่วไปของลำดับเลขคณิต} \\ a_n = a_1+\left(n-1\right)d \\ \mathrm{จากโจทย์} n=31, a_1=-\frac{1}{20} \\ \mathrm{จะได้} d = \left(-\frac{1}{30}\right)-\left(-\frac{1}{20}\right) \\ = -\frac{1}{30}+\frac{1}{20} = \frac{1}{60} \end{gathered}$$

$$\begin{gathered} \mathrm{จะได้} a_{31} = -\frac{1}{20}+\left(31-1\right)\left(\frac{1}{60}\right) \\ a_{31} = -\frac{1}{20}+\frac{1}{2} \\ = \frac{-1+10}{20} \\ \mathrm{ดังนั้น} a_{31} = \frac{9}{20} \end{gathered}$$