a: \(\Leftrightarrow2^{4x}< 2^{28}\)
=>4x<28
hay x<7
b: \(\Leftrightarrow5^{3x+3}>=5^{18}\)
=>3x+3>=18
=>3x>=15
hay x>=5
`a) 16^x< 128^4`
`=>` \(2^{4x}< 2^{7.4}\)
`=>` \(2^{4x}< 2^{28}\)
`=> 4x < 28`
`=> x < 28 : 4`
`=> x < 7`
`=> x` ∈ `{0 ; 1 ; 2 ; 3 ; 4 ; 5 ; 6}`
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`5^x xx 5^(x+1) xx 5^(x+2)` ≥ `100...0 : 2^18` ( 18 số 0)
`=> 5^x xx 1 + 5^x xx 5 xx 5^x xx 5^2` ≥ `10^18 : 2^18`
`=> 5^x xx (1 + 5 + 5^2)` ≥ (10 : 2)^18`
`=> 5^x xx (5^0 + 5^1 + 5^2)` ≥ `5^18`
`=> 5^x xx 5^3` ≥ 5^18`
`=> 5^x` ≥ `5^18 : 5^3`
`=> 5^x ` ≥ 5^15`
`=> x` ∈ `{16 ; 17 ; ...}`
