A = 4 + $4^{2}$ + $4^{3}$ + $4^{4}$ + ... + $4^{100}$
⇒ 4.A= 4 . ( 4+ $4^{2}$ + $4^{3}$ + $4^{4}$ + ... + $4^{100}$ )
⇒ 4A = $4^{2}$+$4^{3}$+$4^{4}$+$4^{5}$+...+$4^{101}$
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A = 4 + $4^{2}$ + $4^{3}$ + $4^{4}$ + ... + $4^{100}$
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4A-A = $4^{101}$ - 4
⇒ 3A = $4^{101}$ - 4
⇒ A = $\dfrac{4^{101} - 4 }{3}$
Chúc bạn học tốt!!