Đáp án:
Bạn tham khảo nhé!
Giải thích các bước giải:
$3,1(2)=3,12222...$
Đặt $A=3,12222....$
$\Rightarrow 10A=31,2222...\\\Rightarrow 100=312,2222...\\\Rightarrow 100A-10A=312,2222...-31,2222...\\\Rightarrow 90A=281\\\Rightarrow A=\dfrac{281}{90}$
Vậy $3,1(2)=\dfrac{281}{90}$.
$5,(21)=5,212121...$
Đặt $A=5,212121...$
$\Rightarrow 100A=521,212121...\\\Rightarrow 100A-A=521,212121...-5,212121...\\\Rightarrow 99A=516\\\Rightarrow A=\dfrac{172}{33}$
Vậy $5,(21)=\dfrac{172}{33}$.
