Đáp án:
Bài 4 :
`723 = 7 xx 100 + 2 xx 10 + 3 xx 1`
`723 = 7 xx 10^2 + 2 xx 10^1 + 3 xx 10^0`
`mnpq = m xx 1000 + n x 100 + p x 10 + q x 1`
`mnpq = m xx 10^3 + n xx 10^2 + p xx 10^1 + q x 10^0`
Bài 5 :
`a, 3^n = 27`
`3^n = 3^3`
`-> n = 3`
`b, 5^n = 625`
`-> 5^n = 5^4`
`-> n = 4`
`c, 12^n = 144`
`-> 12^n = 12^2`
`-> n = 2`
Bài 6 :
`a, 2^n . 16 = 128`
`-> 2^n = 128 : 16 `
`-> 2^n = 8`
`-> 2^n = 2^3`
`-> n = 3`
`b, 3^n . 9 = 27`
`-> 3^n = 27 : 9`
`-> 3^n = 3`
`-> 3^n = 3^1`
`-> n = 1`
Bài 7 :
`a, (2n+1)^3 = 27`
`-> (2n+1)^3 = 3^3`
`-> 2n+1 = 3`
`-> 2n = 2`
`-> n = 2:2 =1`
`b, (n-2)^2 = (n-2)^4`
`-> (n-2)^4 - (n-2)^2 = 0`
`-> (n-2)^2.((n-2)^2 - 1) = 0`
`->` $\left[\begin{matrix} (n-2)^2 = 0\\ (n-2)^2 - 1 = 0\end{matrix}\right.$
`->` $\left[\begin{matrix} (n-2)^2 = 0^2\\ (n-2)^2 = 1\end{matrix}\right.$
`->` $\left[\begin{matrix} n-2=0\\ (n-2)^2 = (±1)^2\end{matrix}\right.$
`->` $\left[\begin{matrix} n=2\\ n-2 = ±1\end{matrix}\right.$
`->` $\left[\begin{matrix} n=2\\n-2=1\\n-2=-1\end{matrix}\right.$
`->` $\left[\begin{matrix} n=2\\n=3\\n=1\end{matrix}\right.$
Vậy `n in {1,2,3}`
