ข้อสอบ PAT 1 - กุมภาพันธ์ 2563

$$\begin{gathered} \mathrm{จากสูตรลำดับเลขคณิต} \\ a_n = a_1+\left(n-1\right)d \\ a_1+a_3 = 7 \\ a_1+a_1+2d = 7 \\ 2a_1+2d = 7 \dots \overline{)1} \end{gathered}$$

$$\begin{gathered} a_2+a_4+a_6+a_8 = 74 \\ {a}_1+d+a_1+3d+a_1+5d+a_1+7d = 74 \\ 4a_1+16d = 74 \\ 2a_1+8d = 37 \dots \overline{)2} \end{gathered}$$

$$\begin{gathered} \overline{)2} - \overline{)1} ; 6d = 30 \\ d = 5 \mathrm{แทนใน} \overline{)1} \\ 2a_1+2\left(5\right) = 7 \\ a_1 = -\frac{3}{2} \end{gathered}$$