Đáp án:$\sqrt[]{1,3}$
Giải thích các bước giải:
$\sqrt[]{\dfrac{(209-207,6)^2+(208-207,6)^2+(206-207,6)^2+(207-207,6)^2+(208-207,6)^2}{5-1}}$
$=\sqrt[]{\dfrac{1,4^2+0,4^2+(-1,6)^2+(-0,6)^2+0,4^2}{4}}$
$=\sqrt[]{\dfrac{1,4^2+2.0,4^2+1,6^2+0,6^2}{4}}$
$=\sqrt[]{\dfrac{(1,4^2-2.1,4.0,4+0,4^2)+(1,6^2-2.1,6.0,6+0,6^2)+(0,4^2+2.1,4.0,4+2.1,6.0,6)}{4}}$
$=\sqrt[]{\dfrac{(1,4-0,4)^2+(1,6-0,6)^2+((0,4^2+2.1,6.0,2)+(2.1,4.0,4+2.1,6.0,4))}{4}}$
$=\sqrt[]{\dfrac{1^2+1^2+((0,4^2+1,6.0,4)+2.0,4(1,4+1,6))}{4}}$
$=\sqrt[]{\dfrac{2+(0,4.2+2.0,4.3)}{4}}$
$=\sqrt[]{\dfrac{2+8.0,4}{4}}$
$=\sqrt[]{0,5+2.0,4}$
$=\sqrt[]{1,3}$
