Question

Answer 1

Có \(u_n=u_1+\left(n-1\right)\cdot d\) với \(n\ge2\)

\(\Rightarrow u_3=u_1+\left(3-1\right)\cdot d\)

\(\Rightarrow u_3=2+2\cdot1=4\)

B

Answer 2

\(ADCT:u_n=u_1+\left(n-1\right).d\\ \Rightarrow u_3=2+\left(3-1\right).1=4\\ \Rightarrow B\)