Theo Cauchy:

\(3\sqrt{2a-1}=3\sqrt{1\left(2a-1\right)}\le\dfrac{3\left(1+2a-1\right)}{2}=3a\)

\(a\sqrt{5-4a^2}\le\dfrac{a^2+5-4a^2}{2}=\dfrac{5-3a^2}{2}\)

\(A\le3a+\dfrac{5-3a^2}{2}=\dfrac{5-3a^2+6a}{2}=\dfrac{-3\left(a-1\right)^2}{2}+4\le4\)

Vậy \(A_{max}=4\Leftrightarrow x=1\)

bạn có cách nào đoán điểm rơi hay thế ạ , phải thử thôi hay có cách gì khác nữa không v

\(a,a=-\dfrac{3}{2}\)

\(\Rightarrow3\left[2\left(-\dfrac{3}{2}\right)-1\right]+5\left(3+\dfrac{3}{2}\right)=3.\left(-3-1\right)+5.\dfrac{9}{2}=-12+\dfrac{45}{2}=\dfrac{21}{2}\)

\(b,x=2,1\)

\(\Rightarrow25.2,1-4\left(3.2,1-1\right)+7\left(5-2.2,1\right)=52,5-4.5,3+7.0,8=36,9\)

\(c,b=\dfrac{1}{2}\)

\(\Rightarrow12\left(2-3.\dfrac{1}{2}\right)+35.\dfrac{1}{2}-9\left(\dfrac{1}{2}+1\right)=12.\dfrac{1}{2}+\dfrac{35}{2}-9.\dfrac{3}{2}=6+\dfrac{35}{2}-\dfrac{27}{2}=10\)

\(d,a=-0,2\)

\(\Rightarrow4.\left(-0,2\right)^2-2\left(10.\left(-0,2\right)-1\right)+4.\left(-0,2\right)\left(2-\left(-0,2\right)^2\right)\)

\(=4.0,04-2.\left(-3\right)-0,8.1,96\)

\(=0,16+6-1,568\)

\(=4,592\)

a: A=6a-3+15-5a=a+12

Khi a=-3/2 thì A=-3/2+12=10,5

b: B=25x-12x+4+35-8x=5x+39

Khi x=2,1 thì B=10,5+39=49,5

c: C=24-6b+35b-9b-9=20b+15

Khi b=0,5 thì C=10+15=25

d: D=4a^2-20a+2+8a-4a^3=-4a^3+4a^2-12a+2

Khi a=-0,2 thì 

D=-4*(-1/5)^3+4*(-1/5)^2-12*(-1/5)+2=4,592

đợi tý

Đã trả lời rồi còn độ tí đồ ngull

a) ĐKXĐ: a²-1 ≠0 ⇔ (a-1)(a+1)≠0 ⇔\(\left[{}\begin{matrix}a-1\ne0\\a+1\ne0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}a\ne1\\a\ne-1\end{matrix}\right.\)

b) A=\(\dfrac{2a^2}{a^2-1}-\dfrac{a}{a+1}+\dfrac{a}{a-1}\) , a≠1, -1

      =\(\dfrac{2a^2}{\left(a-1\right)\left(a+1\right)}-\dfrac{a\left(a-1\right)}{\left(a-1\right)\left(a+1\right)}+\dfrac{a\left(a+1\right)}{\left(a-1\right)\left(a+1\right)}\)

      =\(\dfrac{2a^2-a\left(a-1\right)+a\left(a+1\right)}{\left(a-1\right)\left(a+1\right)}\)

      =\(\dfrac{2a^2-a^2+a+a^2+a}{\left(a-1\right)\left(a+1\right)}\)

      =\(\dfrac{2a^2+2a}{\left(a-1\right)\left(a+1\right)}\) =\(\dfrac{2a\left(a+1\right)}{\left(a-1\right)\left(a+1\right)}\) =\(\dfrac{2a}{a-1}\)

vậy A =\(\dfrac{2a}{a-1}\) với a≠1,-1.

c) Có:A= \(\dfrac{2a}{a-1}\) = \(\dfrac{2a-2+2}{a-1}=\dfrac{2\left(a-1\right)+2}{a-1}=2+\dfrac{2}{a-1}\)

Để a∈Z thì a-1 ∈ Z ⇒ (a-1) ∈ Ư(2) =\(\left\{1;-1;2;-2\right\}\)

Ta có bảng sau:

Vậy để biểu thức A có giá trị nguyên thì a∈\(\left\{2;0;3\right\}\)

a) ĐKXĐ: \(a\notin\left\{1;-1\right\}\)

b) Ta có: \(A=\dfrac{2a^2}{a^2-1}-\dfrac{a}{a+1}+\dfrac{a}{a-1}\)

\(=\dfrac{2a^2}{\left(a+1\right)\left(a-1\right)}-\dfrac{a\left(a-1\right)}{\left(a+1\right)\left(a-1\right)}+\dfrac{a\left(a+1\right)}{\left(a+1\right)\left(a-1\right)}\)

\(=\dfrac{2a^2-a^2+a+a^2+a}{\left(a+1\right)\left(a-1\right)}\)

\(=\dfrac{2a^2+2a}{\left(a+1\right)\left(a-1\right)}\)

\(=\dfrac{2a\left(a+1\right)}{\left(a+1\right)\left(a-1\right)}\)

\(=\dfrac{2a}{a-1}\)

c) Để A nguyên thì \(2a⋮a-1\)

\(\Leftrightarrow2a-2+2⋮a-1\)

mà \(2a-2⋮a-1\)

nên \(2⋮a-1\)

\(\Leftrightarrow a-1\inƯ\left(2\right)\)

\(\Leftrightarrow a-1\in\left\{1;-1;2;-2\right\}\)

\(\Leftrightarrow a\in\left\{2;0;3;-1\right\}\)

Kết hợp ĐKXĐ, ta được: \(a\in\left\{0;2;3\right\}\)

Vậy: Để A nguyên thì \(a\in\left\{0;2;3\right\}\)

Lời giải:
a.

\(A=\frac{\sqrt{a}(a\sqrt{a}+1)}{a-\sqrt{a}+1}-\frac{\sqrt{a}(2\sqrt{a}+1)}{\sqrt{a}}+1\)

\(=\frac{\sqrt{a}(\sqrt{a}+1)(a-\sqrt{a}+1)}{a-\sqrt{a}+1}-(2\sqrt{a}+1)+1\)

\(=\sqrt{a}(\sqrt{a}+1)-(2\sqrt{a}+1)+1=a-\sqrt{a}\)

b.

$A=a-\sqrt{a}=(\sqrt{a}-0,5)^2-0,25\geq -0,25$ với mọi $a>0$

Vậy $A_{\min}=-0,25$ khi $\sqrt{a}-0,5=0$

$\Leftrightarrow a=0,25$

cho mình hỏi làm sao để tách\(a\sqrt{a}+1=\left(\sqrt{a}+1\right)\left(a-\sqrt{a}+1\right)\)

Áp dụng bất đẳng thức: \(\dfrac{1}{a}+\dfrac{1}{b}\ge\dfrac{4}{a+b}\)

\(\Leftrightarrow\left(a+b\right)^2\ge4ab\) \(\Leftrightarrow a^2+2ab+b^2\ge4ab\Leftrightarrow a^2-2ab+b^2\ge0\Leftrightarrow\left(a-b\right)^2\ge0\left(đúng\right)\)

\(\dfrac{1}{2a+b+c}=\dfrac{1}{4}.\dfrac{4}{2a+b+c}\le\dfrac{1}{4}\left(\dfrac{1}{2a}+\dfrac{1}{b+c}\right)\le\dfrac{1}{4}\left[\dfrac{1}{2a}+\dfrac{1}{4}\left(\dfrac{1}{b}+\dfrac{1}{c}\right)\right]=\dfrac{1}{8}\left(\dfrac{1}{a}+\dfrac{1}{2b}+\dfrac{1}{2c}\right)\)

CMTT \(\Leftrightarrow\left\{{}\begin{matrix}\dfrac{1}{a+2b+c}\le\dfrac{1}{8}\left(\dfrac{1}{2a}+\dfrac{1}{b}+\dfrac{1}{2c}\right)\\\dfrac{1}{a+b+2c}\le\dfrac{1}{8}\left(\dfrac{1}{2a}+\dfrac{1}{2b}+\dfrac{1}{c}\right)\end{matrix}\right.\)

\(\Rightarrow M=\dfrac{1}{2a+b+c}+\dfrac{1}{a+2b+c}+\dfrac{1}{a+b+2c}\le\dfrac{1}{8}\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}+\dfrac{2}{2a}+\dfrac{2}{2b}+\dfrac{2}{2c}\right)=\dfrac{1}{4}\left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)=\dfrac{1}{4}.4=1\)

\(minM=1\Leftrightarrow a=b=c=\dfrac{3}{4}\)

Sửa lại \(minM=1\rightarrow maxM=1\)

a: \(A=a+\sqrt{a}-2\sqrt{a}-1+1=a-\sqrt{a}\)

trình bày rõ ràng ra bạn còn câu b nữa