$2^{30}$ và $3^{20}$
Ta có :$2^{30} = 2^{3.10}=\left ( 2^{3} \right )^{10}=8^{10}3^{20}= 3^{2.10}=\left ( 3^{2} \right )^{10}=9^{10}$
`⇒2^{30} < 3^{20}`
`A = 1 + 5 + 5^2 +...+ 5^30`
`⇒5A = 5 + 5^2 +...+ 5^31`
`⇒5A - A = ( 5 + 5^2 +...+ 5^31 ) - ( 1 + 5 + 52 +...+ 5^30 )`
`⇒4A = 5^31 - 1
`⇒A = (5^31−1)/4`