ข้อสอบคณิต O-NET - 2551

$$\begin{gathered} \mathrm{จากสูตร} \mathrm{พจน์ทั่วไป} \\ a_n = a_1{r}^{n-1} : \mathrm{ลำดับเรขาคณิต} \\ a_n = a_1+\left(n-1\right)d : \mathrm{ลำดับเลขคณิต} \end{gathered}$$

$$\begin{gathered} \mathrm{จากโจทย์} \\ \left(ก\right) \mathrm{จะได้} a_1+a_1{r}^2, a_{1}r+a_1{r}^3, a_1{r}^2+a_1{r}^4, \dots \mathrm{ถูก} \\ \times r \times r \end{gathered}$$

$$\begin{gathered} \left(ข\right) \mathrm{จะได้} a_1·a_{1}r, a_{1}r·a_1{r}^2, a_1{r}^2·a_1{r}^3, \dots \\ {a_1}^{2}r , {a_1}^2{r}^3 , {a_1}^2{r}^5 , \dots \mathrm{ถูก} \\ \times r^2 \times r^2 \end{gathered}$$

$$\begin{gathered} \left(ค\right) \mathrm{จะได้} \frac{1}{a_1} , \frac{1}{a_{1}r} , \frac{1}{a_1{r}^2} , \dots \mathrm{ถูก} \\ \times \frac{1}{r} \times \frac{1}{r} \\ \mathrm{ดังนั้น} \mathrm{ถูกทั้ง} 3 \mathrm{ข้อ} \end{gathered}$$