Số thứ nhất `=1+4x1`

Số thứ hai `=1+4x2`

Số thứ ba `=1+4x3`

...

Số thứ `23` `=1+4x23=93`

Số hạng thứ 23 của tổng a là:

     ( 23-1) x 4 + 5 = 93

C

c, 48

`x-135:45=200`

`=>x-3=200`

`=>x=200+3=203`

2006 á

\(\left(x+1\right)^4=\left(2x\right)^4=\left(\pm2x\right)^4\\ =>\left[{}\begin{matrix}x+1=2x\\x+1=-2x\end{matrix}\right.=>\left[{}\begin{matrix}x=1\left(TM\right)\\x=-\dfrac{1}{3}\left(KTM\right)\end{matrix}\right.\)

\(\left(2x-1\right)^5=x^5\\ =>2x-1=x\\ =>2x-x=1\\ =>x=1\left(TM\right)\)

\(\dfrac{5}{3}x+\dfrac{7}{2}=\dfrac{-2}{-3}\\ =>\dfrac{5}{3}x=\dfrac{2}{3}-\dfrac{7}{2}=-\dfrac{17}{6}\\ =>x=-\dfrac{17}{6}:\dfrac{5}{3}=-\dfrac{17}{10}\)

\(\dfrac{5}{3}x+\dfrac{7}{2}=\dfrac{-2}{-3}\)

\(\dfrac{5}{3}x+\dfrac{7}{2}=\dfrac{2}{3}\)

\(\dfrac{5}{3}x=\dfrac{2}{3}-\dfrac{7}{2}\)

\(\dfrac{5}{3}x=\dfrac{-17}{6}\)

\(x=\dfrac{-17}{6}:\dfrac{5}{3}\)

\(x=\dfrac{-17}{6}.\dfrac{3}{5}\)

\(x=-\dfrac{51}{30}\)

\(3\times x-45=15\\ =>3\times x=15+45=60\\ =>x=\dfrac{60}{3}=20\)

3.x-45=15

3x=15+45

3x=60

x=60:3

x=20

9979-3050-9978+3051+x=3

=>2+x=3

=>x=3-2

=>x=1

\(\dfrac{72\overline{abc}}{\overline{abc}}=76\\ =>\dfrac{72000}{\overline{abc}}+\dfrac{\overline{abc}}{\overline{abc}}=76\\ =>\dfrac{72000}{\overline{abc}}+1=76\\ =>\dfrac{72000}{\overline{abc}}=75\\ =>\overline{abc}=72000:75=960\)

$72abc:abc=76$

$\Rightarrow 72000+abc=76\times abc$

$\Rightarrow 72000=76\times abc-abc$

$\Rightarrow 72000=abc\times (76-1)$

$\Rightarrow abc\times 75=72000$

$\Rightarrow abc=72000:75$

$\Rightarrow abc=960$

Vậy $abc=960$