a,\(A=\left(x+1\right)\left(x+2\right)\left(x+4\right)\left(x+5\right)=\left(x^2+6x+5\right)\left(x^2+6x+8\right)\)

đặt \(x^2+6x+5=t=>t\left(t+3\right)=t^2+3t=t^2+2.\dfrac{3}{2}t+\dfrac{9}{4}-\dfrac{9}{4}\)

\(=\left(t+\dfrac{3}{2}\right)^2-\dfrac{9}{4}\ge-\dfrac{9}{4}< =>t=\dfrac{-3}{2}\)

\(=>A\)\(=-\dfrac{3}{2}\left(-\dfrac{3}{2}+3\right)=-2,25\)

Vậy Min A\(=-2,25\)

b,\(B=-x^2-4x-9y^2-6y-6\)

\(=-\left(x^2+4x+4\right)-\left(3y\right)^2-2.3y-1-1\)

\(=-\left(x+2\right)^2-\left(3y+1\right)^2-1\le-1\)

dấu"=' xảy ra\(< =>x=-2,y=-\dfrac{1}{3}\)

a.

$(x+1)(x+2)(x+4)(x+5)=(x+1)(x+5)(x+2)(x+4)=(x^2+6x+5)(x^2+6x+8)$

$=a(a+3)$ với $a=x^2+6x+5$

$=a^2+3a=(a^2+3a+\frac{9}{4})-\frac{9}{4}$

$=(a+\frac{3}{2})^2-\frac{9}{4}$

$=(x^2+6x+\frac{13}{2})^2-\frac{9}{4}\geq \frac{-9}{4}$

Vậy gtnn của biểu thức là $\frac{-9}{4}$. Giá trị này đạt tại $x^2+6x+\frac{13}{2}=0$

$\Leftrightarrow x=\frac{-6\pm \sqrt{10}}{2}$

a: A=x^2-6x+9+2=(x-3)^2+2>=2

Dấu = xảy ra khi x=3

b: B=x^2-20x+100+1=(x-10)^2+1>=1

Dấu = xảy ra khi x=10

d: C=x^2-16x+8+3

=(x-4)^2+3>=3

Dấu = xảy ra khi x=4

Bài 2 :

\(A=4x^2-2.2x.2+4+1\)

\(=\left(2x-2\right)^2+1\)

Thấy : \(\left(2x-2\right)^2\ge0\)

\(A=\left(2x-2\right)^2+1\ge1\)

Vậy \(MinA=1\Leftrightarrow x=1\)

\(B=\left(5x\right)^2-2.5x.1+1-4\)

\(=\left(5x-1\right)^2-4\)

Thấy : \(\left(5x-1\right)^2\ge0\)

\(\Rightarrow B=\left(5x-1\right)^2-4\ge-4\)

Vậy \(MinB=-4\Leftrightarrow x=\dfrac{1}{5}\)

\(C=\left(7x\right)^2-2.7x.2+4-5\)

\(=\left(7x-2\right)^2-5\)

Thấy : \(\left(7x-2\right)^2\ge0\)

\(\Rightarrow C=\left(7x-2\right)^2-5\ge-5\)

Vậy \(MinC=-5\Leftrightarrow x=\dfrac{2}{7}\)

\(1.\)

\(A=-x^2-10x+1=-\left(x^2+10x-1\right)\)

\(=-\left(x^2+2.5x+5^2-5^2-1\right)=-\left[\left(x+5\right)^2-26\right]\)

\(=-\left(x+5\right)^2+26\le26\) dấu "=" xảy ra<=>x=-5

\(B=-4x^2-6x-5=-4\left(x^2+\dfrac{6}{4}x+\dfrac{5}{4}\right)\)

\(=-4\left(x^2+2.\dfrac{3}{4}x+\dfrac{9}{16}+\dfrac{11}{16}\right)\)\(=-4\left[\left(x+\dfrac{3}{2}\right)^2+\dfrac{11}{6}\right]\le-\dfrac{11}{4}\)

\(C=-16x^2+8x-1=-16\left(x^2-\dfrac{1}{2}x+\dfrac{1}{16}\right)\)

\(=-16\left(x^2-2.\dfrac{1}{4}x+\dfrac{1}{16}\right)=-16\left(x-\dfrac{1}{4}\right)^2\le0\)

dấu"=" xảy ra<=>x=1/4

Lời giải:
ĐKXĐ: $x>0$

Áp dụng BĐT Cô-si: $x+9\geq 2\sqrt{9x}=6\sqrt{x}$

$\Rightarrow A=\frac{x+9}{6\sqrt{x}}=\frac{6\sqrt{x}}{6\sqrt{x}}=1$

Vậy $A_{\min}=1$ khi $x=9$

Min của A là 99 khi (x;y)=(2;3).

Chúc abh học tốt.

\(A=\left(x+\dfrac{4}{x}\right)+5\left(\dfrac{y}{3}+\dfrac{3}{y}\right)+17\left(x+y\right)\)

\(A\ge2\sqrt{\dfrac{4x}{x}}+5.2\sqrt{\dfrac{3y}{3y}}+17.5=99\)

Dấu "=" xảy ra khi \(\left(x;y\right)=\left(2;3\right)\)

1. Thu gọn biểu thức - Hoc24 làm rồi mà bạn?

1.

a) \(=x^2-6x+9+3x^2-15x=4x^2-21x+9\)

b) \(=9x^2+12x+4-x^2+9=8x^2+12x+13\)

2.

a) \(\Leftrightarrow x^2+8x+16-x^2+4-5=0\\ \Leftrightarrow8x=-15\\ \Leftrightarrow x=-\dfrac{15}{8}\)

b) \(\Leftrightarrow9x^2-6x+1-8x^2+12x-2x+3-5-x^2=0\\ \Leftrightarrow4x=1\\ \Leftrightarrow x=\dfrac{1}{4}\)

1,a,=x2−6x+8+3x2−15x=4x2−21x+8b,=9x2+12x+4−x2+9=8x2+12x+132,a,⇔x2+8x+16−x2+4=5⇔8x=−15⇔x=−158b,⇔9x2−6x+1−8x2−2x+12x+3−x2=5⇔4x=1⇔x=14

\(a,A=\left(x^2+5x+\dfrac{25}{4}\right)+\dfrac{7}{4}=\left(x+\dfrac{5}{2}\right)^2+\dfrac{7}{4}\ge\dfrac{7}{4}\\ A_{min}=\dfrac{7}{4}\Leftrightarrow x=-\dfrac{5}{2}\\ b,B=x^2-6x+9-9=\left(x-3\right)^2-9\ge9\\ B_{min}=-9\Leftrightarrow x=3\)

\(A=\sqrt{x-2}+\sqrt{4-x}\ge\sqrt{x-2+4-x}=\sqrt{2}\)

\(A_{min}=\sqrt{2}\) khi \(\left[{}\begin{matrix}x=2\\x=4\end{matrix}\right.\)

\(y=4x^2+\dfrac{9}{x^2}-3\ge2\sqrt{\dfrac{36x^2}{x^2}}-3=9\)

\(y_{min}=9\) khi \(x^2=\dfrac{3}{2}\)

\(P=\dfrac{x-1}{4}+\dfrac{1}{x-1}+\dfrac{1}{4}\ge2\sqrt{\dfrac{x-1}{4\left(x-1\right)}}+\dfrac{1}{4}=\dfrac{5}{4}\)

\(P_{min}=\dfrac{5}{4}\) khi \(x=\dfrac{3}{2}\)