\(5^x-5^{x-1}-5^{x-2}=2375\\ \Leftrightarrow5^x-\dfrac{5^x}{5}-\dfrac{5^x}{25}=2375\\ \Leftrightarrow5^x\left(1-\dfrac{1}{5}-\dfrac{1}{25}\right)=2375\\ \Leftrightarrow5^x.\dfrac{19}{25}=2375\\ \Leftrightarrow5^x=3125\\ \Leftrightarrow5^x=5^5\\ \Leftrightarrow x=5\)

\(5^x-5^{x-1}-5^{x-2}=2375\)

\(\Rightarrow5^{x-2}\left(5^2-5^1-1\right)=2375\)

\(\Rightarrow5^{x-2}.19=2375\)

\(\Rightarrow5^{x-2}=125=5^3\Rightarrow x-2=3\Rightarrow x=5\)

Ta có: \(9\cdot5^{x-2}+7\cdot5^{x-1}-5^x=2375\)

\(\Leftrightarrow5^x\cdot\dfrac{9}{25}+5^x\cdot\dfrac{7}{5}-5^x\cdot1=2375\)

\(\Leftrightarrow5^x\cdot\dfrac{19}{25}=2375\)

\(\Leftrightarrow5^x=2375:\dfrac{19}{25}=2375\cdot\dfrac{25}{19}=3125\)

\(\Leftrightarrow5^x=5^5\)

hay x=5

Vậy: x=5

`2375 xx (72 : 8) + 49`

`= 2375 xx 9 + 49`

`= 21424`

_____________________

`2375 xx (72 : 9) + 49`

`= 2375 xx 8 + 49`

`= 19049`

Thay n=8, ta có:

2375 x ( 72 : 8 ) + 49

= 21424

Thay n=9, ta có:

2375 x ( 72 : 9 ) + 49

=19049

n=8

2375 x (72:8) +49

= 2375 x 9 +49

= 21375 + 49

=21424

n=9

2375 x (72:9) + 49

= 2375 x 8 +49

= 19000+49

=19049

c) Những số chia hết cho 5 là : 2375; 2790     Đ

Đáp án là B

=100000-(47.(75+24+1)       b)    =276.(50+36+4)

=100000-4700                              =276.90

=95300                                         =24840

\(100000-\left(47.75+47.24+47\right)\)

\(=100000-\left(47.75+47.24+47.1\right)\)

\(=100000-\left[47.\left(75+24+1\right)\right]\)

\(=100000-47.100\)

\(=100000-47000\)

\(=53000\)

\(276.50+276.36+1104\)

\(=276.50+276.36+276.4\)

\(=276.\left(50+36+4\right)\)

\(=276.90\)

\(=24840\)

100000-(47.75+47.24+47)

=100000-[47.(75+24+1)]

=100000-[47.(99+1)]

=100000-[47.100]

=100000-4700

=96300

276x50+276x36+1104

=276(50+36)+276.4

=276.86+276.4

=276.(86+4)

=276.90

=24840

Đáp án là C

Theo thứ tự từ bé đến lớn: 1780;  1782 ;  2274 ; 2375