Toan lop 6

A

acteyro123

H

hocvuima

Ta đặt tử số là $B$, mẫu số là $C$
Ta có:
$C=\dfrac{19}{1}+\dfrac{18}{2}+...+\dfrac{2}{18}+ \dfrac{1}{19}$
$C=1+\dfrac{18}{2}+...+1+\dfrac{2}{18}+1+\dfrac{1}{19}+1$
$C=\dfrac{20}{2}+...+\dfrac{20}{18}+\dfrac{20}{19}+\dfrac{20}{20}$
$C=20.(\dfrac{1}{2}+...+\dfrac{1}{18}+\dfrac{1}{19}+\dfrac{1}{20})$
$C=20.B$
\Rightarrow $A=\dfrac{B}{C}=\dfrac{B}{B.20}=\dfrac{1}{20}$
 
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