\(\text{A= 73 - (35 + 1𝑎) : 23}\)

thay \(a=45\) vào biểu thức ta có:

\(A=73-\left(35+45\right):23\)

\(=73-\frac{80}{23}=\frac{1599}{23}\)

b)\(73-\left(35+1a\right):23=1715\)

\(\left(35+1a\right):23=-1642\)

\(35+a=-37766\)

\(a=-37801\)

mình ko hiểu đc câu trả lời của bạn 

mình sorry nhé

\(C=\left(x-5\right)^2+10\)

Ta có: \(\left(x-5\right)^2\ge0\forall x\)

\(\Rightarrow C=\left(x-5\right)^2+10\ge10\forall x\)

Dấu \("="\) xảy ra khi: \(x-5=0\Leftrightarrow x=5\)

Vậy \(Min_C=10\) khi \(x=5\).

∀x là gì vậy bạn chracter debate

Đặt A = 1.2 + 2.3 + 3.4 + ...... + 99.100

 3A=1.2.3 - 1.2.3 + 2.3.4 - 2.3.4  + .....+99.100.101

3A=99.100.101

A=99.100.101/3=333300

Đặt A = 1.2 + 2.3 + 3.4 + ...... + 99.100

 3A=1.2.3 - 1.2.3 + 2.3.4 - 2.3.4  + .....+99.100.101

3A=99.100.101

A=99.100.101/3=333300

Đặt A = 1.2 + 2.3 + 3.4 + ...... + 99.100

3A=1.2.(3-0) + 2.3.(4-1) +.....+99.100.(101-98)

 3A=1.2.3 - 1.2.3 + 2.3.4 - 2.3.4  + .....+99.100.101

3A=99.100.101

A=99.100.101/3=333300

\(A=|x-2012|+|x-2013|=|x-2012|+|2013-x|\ge|x-2012+2013-x|=1\)

Dấu = xảy ra \(< =>2012\le x\le2013\)

\(|x-2012|+|x-2013|\)    

\(=|x-2012|+|-\left(2013-x\right)|\)   

\(=|x-2012|+|2013-x|\)    

Ta có 

\(|x-2012|+|2013-x|\ge|x-2012+2013-x|\)    

\(|x-2012|+|2013-x|\ge1\)   

Dấu = xảy ra  

\(\Leftrightarrow\left(x-2012\right)\left(2013-x\right)\ge0\)    

TH 1 : 

\(\hept{\begin{cases}x-2012\ge0\\2013-x\le0\end{cases}}\)    

\(\hept{\begin{cases}x\ge2012\\-x\ge-2013\end{cases}}\)    

\(\hept{\begin{cases}x\ge2012\\x\le2013\end{cases}}\)   \(\Rightarrow2012\le x\le2013\)   

TH 2 

\(\hept{\begin{cases}x-2012\le0\\2013-x\le0\end{cases}}\)   

\(\hept{\begin{cases}x\le2012\\-x\le-2013\end{cases}}\)   

\(\hept{\begin{cases}x\le2012\\x\ge2013\end{cases}}\)    \(\Rightarrow x=\varnothing\)    

Vậy min A = 1 khi và chỉ khi \(2012\le x\le2013\)

\(A=\dfrac{5x^2}{x^2}-\dfrac{x}{x^2}+\dfrac{1}{x^2}=\dfrac{1}{x^2}-\dfrac{1}{x}+5=\left(\dfrac{1}{x^2}-\dfrac{1}{x}+\dfrac{1}{4}\right)+\dfrac{19}{4}=\left(\dfrac{1}{x}-\dfrac{1}{2}\right)^2+\dfrac{19}{4}\ge\dfrac{19}{4}\)

\(A_{min}=\dfrac{19}{4}\) khi \(\dfrac{1}{x}=\dfrac{1}{2}\Rightarrow x=2\)

Đặt A = 1.2 + 2.3 + 3.4 + ...... + 99.100

3A= 3.(1.2 + 2.3 + 3.4 + ..... +99.100)

3A=1.2.(3-0) + 2.3.(4-1) +.....+99.100.(101-98)

 3A=1.2.3 - 1.2.3 + 2.3.4 - 2.3.4  + .....+99.100.101

3A=99.100.101

A=99.100.101/3=333300

đặt A = 1.2 + 3.4 + 4.5 +...+ 99.100

A=1.2+2.3+3.4+4.5+...+99.100

=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+99.100.3

=1.2.3+2.3.﴾4‐1﴿+3.4.﴾5‐2﴿+4.5.﴾6‐3﴿+...+99.100.﴾101‐98﴿

=1.2.3+2.3.4‐1.2.3+3.4.5‐2.3.4+4.5.6‐3.4.5+...+99.100.101‐98.99.100

=1.2.3‐1.2.3+2.3.4‐2.3.4+3.4.5‐3.4.5+4.5.6‐4.5.6+...+99.100.101

=99.100.101=999900

=>A=999900:3=333300

Vậy A=333300

đặt A = 1.2 + 3.4 + 4.5 +...+ 99.100

A=1.2+2.3+3.4+4.5+...+99.100

=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+99.100.3

=1.2.3+2.3.﴾4‐1﴿+3.4.﴾5‐2﴿+4.5.﴾6‐3﴿+...+99.100.﴾101‐98﴿

=1.2.3+2.3.4‐1.2.3+3.4.5‐2.3.4+4.5.6‐3.4.5+...+99.100.101‐98.99.100

=1.2.3‐1.2.3+2.3.4‐2.3.4+3.4.5‐3.4.5+4.5.6‐4.5.6+...+99.100.101

=99.100.101=999900

=>A=999900:3=333300

Vậy A=333300