Đáp án:
$a) A = 3+3^2+3^3+........+3^{2020}$
$3A = 3^2 + 3^3 + 3^4 + ... + 3^{2021}$
$3A - A = (3^2 + 3^3 + 3^4 + ... + 3^{2021}) - (3+3^2+3^3+........+3^{2020})$
$2A = 3^2 + 3^3 + 3^4 + ... + 3^{2021} - 3-3^2-3^3-........-3^{2020}$
$2A = 3^{2021} - 3$
$\Rightarrow A = \frac {3^{2021} - 3} {2}$
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$b) B = 1+5^2+5^3+........+5^{2020}$
$5B = 5(5^2+5^3+........+5^{2020})$
$5B = 5^3 + 5^4 + ..... + 5^{2021}$
$5B - B = (5^3 + 5^4 + ..... + 5^{2021}) - (1+5^2+5^3+........+5^{2020})$
$4B = 5^3 + 5^4 + ..... + 5^{2021} - 1 -5^2 -5^3 - ..... - 5^{2020}$
$\Rightarrow B = \frac {5^{2021} - 1} {4}$
$\text{Xin ctlhn ạ!!!}$
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