a_2+a_5=7
a_3+a_8=11
…
a_2=a_1+r
a_5=a_1+4r
a_3=a_1+2r
a_8=a_1+7r
…
\left \{ {{a_2+a_5=7} \atop {a_3+a_8=11}} \right.
\left \{ {{a_1+r+a_1+4r=7} \atop {a_1+2r+a_1+7r=11}} \right.
\left \{ {{2a_1+5r=7} \atop {2a_1+9r=11}} \right. *(-1)
\left \{ {{2a_1+5r=7} \atop {-2a_1-9r=-11}} \right.
…
-4r=-4
r=1
2a_1+5*1=7
2a_1=2
a_1=1
\left \{ {{a_1=1} \atop {r=1}} \right.