giai toan

C

cobematbuon

goi bieu thuc tren la` A
=> A= 1+5^2+5^3+....+5^50
=>5A =5+5^2+5^3+......+5^51
=> 5A =(1+5+5^2+5^3+......+5^50)+(5^51)-1
=>5A=A +(5^51)-1
=>4A=(5^51)-1
=>A=[(5^51)-1]/4
 
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