Question

÷a₁,a₂,a₃,.... a₁=1 a₂=3 a₇=?

Answer 1

a7=a1+6r
=>1+6×2
=>1+12
=>a7=13

Answer 2

$\displaystyle a_1=1,~a_2=3,~a_7=? \\ a_2=3 \Rightarrow a_{2-1}+r=3 \Rightarrow a_1+r=3 \Rightarrow 1+r=3 \Rightarrow r=3-1 \Rightarrow r=2 \\ a_7=a_{7-1}+r \Rightarrow a_7=a_6+r \Rightarrow a_7=a_1+6r \Rightarrow a_7=1+6 \cdot 2 \Rightarrow \\ \Rightarrow a_7=1+12 \Rightarrow \boxed{a_7=13}$