[tex]\displaystyle a_1=6,~a_2=12,~S_{10}=? \\ a_2=12 \Rightarrow a_{2-1}+r=12 \Rightarrow a_1+r=12 \Rightarrow 6+r=12 \Rightarrow \\ \Rightarrow r=12-6 \Rightarrow r=6 \\ S_{10}= \frac{2 \cdot 6+(10-1) \cdot 6}{\not2} \cdot \not10 \\ S_{10}=(12+9 \cdot 6) \cdot 5 \\ S_{10}=(12+54) \cdot 5 \\ S_{10}=66 \cdot 5 \\ S_{10}=330[/tex]
