tính tổng help me !!!

H

huutho2408

tớ thử làm nhé

S=7+77+777+...77....7
9S/7=9+99+999+99....9

=(10-1)+([tex] 10^2 -1[/tex])+...+([tex] 10^n -1[/tex])

=(10+[tex] 10^2 [/tex]+...+[tex] 10^n [/tex])-n

=[10([tex] 10^n -1[/tex])/9]-n

=[tex]\frac{10*10^n -10-9n}{9}[/tex]
suy ra

S=[tex]\frac{7(10*10^n -10-9n)}{81}[/tex]
 
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