Toán Lớp 6: (3^x – 1) : 2 = 1 + 3 + … + 3^2020
Giải hộ mk với
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Toán Lớp 6: (3^x – 1) : 2 = 1 + 3 + … + 3^2020
Giải hộ mk với
Comments ( 1 )
Đặt A=1+3+…+3^2020
=>3A=3(1+3+…+3^2020)
=>3A=3+3²+…+3^2021
=>3A-A=(3+3²+…+3^2021)-(1+3+3²+…+3^2020)
=>2A=3^2021 -1
=>A=(3^2021 -1)/2
=>(3^x -1):2=(3^2021 -1)/2
=>3^x -1 =(3^2021 -1)/2 . 2
=>3^x – 1=3^2021 -1
=>3^x -1 +1=3^2021 -1+1
=>3^x =3^2021
=>x=2021
Vậy x=2021