a) Gọi số cần tìm là `a :``( a \in NN , a \neq 1 )`
`166 - 5 \vdots a`
`51 + 5 \vdost a`
`=> a \in ƯC( 161,56) = \{ 23 \}`
Vậy `a = 23`
`b) E = 2 + 2^2 + 2^3 + 2^4 + ... + 2^{100}`
`=> 2E = 2^2 + 2^3 + 2^4 + 2^5 + ... + 2^{101}`
`=> 2E - E = ( 2^2 + 2^3 + 2^4 + 2^5 + ... + 2^{101} ) - ( 2 + 2^2 + 2^3 + 2^4 + ... + 2^{100} )`
`=> E = 2^{101} - 2`