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NV
Nguyễn Việt Lâm
Giáo viên
10 tháng 3 2023
Đặt \(a=\dfrac{1}{x};b=\dfrac{1}{y};c=\dfrac{1}{z}\Rightarrow xyz=1\) và \(x;y;z>0\)
Gọi biểu thức cần tìm GTNN là P, ta có:
\(P=\dfrac{1}{\dfrac{1}{x^3}\left(\dfrac{1}{y}+\dfrac{1}{z}\right)}+\dfrac{1}{\dfrac{1}{y^3}\left(\dfrac{1}{z}+\dfrac{1}{x}\right)}+\dfrac{1}{\dfrac{1}{z^3}\left(\dfrac{1}{x}+\dfrac{1}{y}\right)}\)
\(=\dfrac{x^3yz}{y+z}+\dfrac{y^3zx}{z+x}+\dfrac{z^3xy}{x+y}=\dfrac{x^2}{y+z}+\dfrac{y^2}{z+x}+\dfrac{z^2}{x+y}\)
\(P\ge\dfrac{\left(x+y+z\right)^2}{y+z+z+x+x+y}=\dfrac{x+y+z}{2}\ge\dfrac{3\sqrt[3]{xyz}}{2}=\dfrac{3}{2}\)
\(P_{min}=\dfrac{3}{2}\) khi \(x=y=z=1\) hay \(a=b=c=1\)
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HQ
Hà Quang Minh
Giáo viên
10 tháng 9 2023
\({x^2} = {4^2} + {2^2} = 20 \Rightarrow x = 2\sqrt 5 \)
\({y^2} = {5^2} - {4^2} = 9 \Leftrightarrow y = 3\)
\({z^2} = {\left( {\sqrt 5 } \right)^2} + {\left( {2\sqrt 5 } \right)^2} = 25 \Rightarrow z = 5\)
\({t^2} = {1^2} + {2^2} = 5 \Rightarrow t = \sqrt 5 \)
NV
Nguyễn Việt Lâm
Giáo viên
16 tháng 1 2024
a.
\(A=\left(\dfrac{\left(x-1\right)\left(x^2+x+1\right)}{x\left(x-1\right)}+\dfrac{\left(x-2\right)\left(x+2\right)}{x\left(x-2\right)}+\dfrac{x-2}{x}\right):\dfrac{x+1}{x}\)
\(=\left(\dfrac{x^2+x+1}{x}+\dfrac{x+2}{x}+\dfrac{x-2}{x}\right):\dfrac{x+1}{x}\)
\(=\left(\dfrac{x^2+3x+1}{x}\right).\dfrac{x}{x+1}\)
\(=\dfrac{x^2+3x+1}{x+1}\)
2.
\(x^3-4x^3+3x=0\Leftrightarrow x\left(x^2-4x+3\right)=0\)
\(\Leftrightarrow x\left(x-1\right)\left(x-3\right)=0\)
\(\Rightarrow\left[{}\begin{matrix}x=0\left(loại\right)\\x=1\left(loại\right)\\x=3\end{matrix}\right.\)
Với \(x=3\Rightarrow A=\dfrac{3^2+3.3+1}{3+1}=\dfrac{19}{4}\)
AH
Akai Haruma
Giáo viên
3 tháng 2 2024
Bài 4:
a. Vì $\triangle ABC\sim \triangle A'B'C'$ nên:
$\frac{AB}{A'B'}=\frac{BC}{B'C'}=\frac{AC}{A'C'}(1)$ và $\widehat{ABC}=\widehat{A'B'C'}$
$\frac{DB}{DC}=\frac{D'B'}{D'C}$
$\Rightarrow \frac{BD}{BC}=\frac{D'B'}{B'C'}$
$\Rightarrow \frac{BD}{B'D'}=\frac{BC}{B'C'}(2)$
Từ $(1); (2)\Rightarrow \frac{BD}{B'D'}=\frac{BC}{B'C'}=\frac{AB}{A'B'}$
Xét tam giác $ABD$ và $A'B'D'$ có:
$\widehat{ABD}=\widehat{ABC}=\widehat{A'B'C'}=\widehat{A'B'D'}$
$\frac{AB}{A'B'}=\frac{BD}{B'D'}$
$\Rightarrow \triangle ABD\sim \triangle A'B'D'$ (c.g.c)
b.
Từ tam giác đồng dạng phần a và (1) suy ra:
$\frac{AD}{A'D'}=\frac{AB}{A'B'}=\frac{BC}{B'C'}$
$\Rightarrow AD.B'C'=BC.A'D'$
Bài 1:
\(a,\)
\(P=2\left(2x-1\right)^2-\left(x+1\right)^2+3\)
\(=2\left(4x^2-4x+1\right)-\left(x^2+2x+1\right)+3\)
\(=8x^2-8x+2-x^2-2x-1+3\)
\(=7x^2-10x+4\)
\(=7\left(x^2-\dfrac{10}{7}x+\dfrac{4}{7}\right)\)
\(=7\left[x^2-2.x.\dfrac{5}{7}+\left(\dfrac{5}{7}\right)^2-\left(\dfrac{5}{7}\right)^2+\dfrac{4}{7}\right]\)
\(=7\left[\left(x-\dfrac{5}{7}\right)^2+\dfrac{3}{49}\right]\)
\(=7\left(x-\dfrac{5}{7}\right)^2+\dfrac{3}{7}\)
Vì \(7\left(x-\dfrac{5}{7}\right)^2\ge0\forall x\)
\(\Rightarrow7\left(x-\dfrac{5}{7}\right)^2+\dfrac{3}{7}\ge\dfrac{3}{7}\forall x\)
\(\Rightarrow P_{min}=\dfrac{3}{7}\Leftrightarrow7\left(x-\dfrac{5}{7}\right)^2=0\Leftrightarrow x=\dfrac{5}{7}\)
\(b,\)
\(Q=3\left(x+2\right)^2-\left(x-2\right)\left(x+2\right)\)
\(=3\left(x^2+4x+4\right)-\left(x^2-4\right)\)
\(=3x^2+12x+12-x^2+4\)
\(=2x^2+12x+16\)
\(=2\left(x^2+6x+8\right)\)
\(=2\left(x^2+6x+9-1\right)\)
\(=2\left[\left(x+3\right)^2-1\right]\)
\(=2\left(x+3\right)^2-2\)
Vì \(2\left(x+3\right)^2\ge0\forall x\)
\(\Rightarrow2\left(x+3\right)^2-2\ge-2\forall x\)
\(\Rightarrow Q_{min}=-2\Leftrightarrow2\left(x+3\right)^2=0\Leftrightarrow x=-3\)
\(c,\)
Tương tự: \(M=\left(x+1\right)^3-\left(x-2\right)^3-5\)
\(=9x^2-9x+4\)
\(=9\left(x^2-x+\dfrac{4}{9}\right)\)
\(=9\left[x^2-2.x.\dfrac{1}{2}+\left(\dfrac{1}{2}\right)^2-\left(\dfrac{1}{2}\right)^2+\dfrac{4}{9}\right]\)
\(=9\left[\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{36}\right]\)
\(=9\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{4}\)
Ta có: \(9\left(x-\dfrac{1}{2}\right)^2+\dfrac{7}{4}\ge\dfrac{7}{4}\forall x\)
\(\Rightarrow M_{min}=\dfrac{7}{4}\Leftrightarrow9\left(x-\dfrac{1}{2}\right)^2=0\Leftrightarrow x=\dfrac{1}{2}\)
Bài 2:
\(a,\)
\(M=-3x^2-4x+1\)
\(=-3\left(x^2+\dfrac{4}{3}x-\dfrac{1}{3}\right)\)
\(=-3\left[x^2+2.x.\dfrac{2}{3}+\left(\dfrac{2}{3}\right)^2-\left(\dfrac{2}{3}\right)^2-\dfrac{1}{3}\right]\)
\(=-3\left[\left(x+\dfrac{2}{3}\right)^2-\dfrac{7}{9}\right]\)
\(=-3\left(x+\dfrac{2}{3}\right)^2+\dfrac{7}{3}\)
Ta có: \(-3\left(x+\dfrac{2}{3}\right)^2\le\dfrac{7}{3}\forall x\)
\(\Rightarrow M_{max}=\dfrac{7}{3}\Leftrightarrow-3\left(x+\dfrac{2}{3}\right)^2=0\Leftrightarrow x=-\dfrac{2}{3}\)
\(b,\)
\(Q=\left(x+2\right)^2-3\left(x-1\right)^2+3\)
\(=-2x^2+10x+4\)
\(=-2\left(x^2-5x-2\right)\)
\(=-2\left[\left(x-\dfrac{5}{2}\right)^2-\dfrac{33}{4}\right]\)
\(=-2\left(x-\dfrac{5}{2}\right)^2+\dfrac{33}{2}\)
Tương tự, \(Q_{max}=\dfrac{33}{2}\Leftrightarrow x=\dfrac{5}{2}\)
\(c,\)
\(P=\left(x-1\right)\left(x+2\right)-2\left(x+3\right)^2\)
\(=-x^2-11x-20\)
\(=-\left(x^2+11x+20\right)\)
\(=-\left(x+\dfrac{11}{2}\right)^2+\dfrac{41}{4}\)
Tương tự, \(Q_{max}=\dfrac{41}{4}\Leftrightarrow x=-\dfrac{11}{2}\)
2:
a: =-3(x^2+4/3x-1/3)
=-3*(x^2+2*x*2/3+4/9-7/9)
=-3(x+2/3)^2+7/3<=7/3
Dấu = xảy ra khi x=-2/3
b: =x^2+4x+4-3x^2+6x-3+3
=-2x^2+10x+4
=-2(x^2-5x-2)
=-2(x^2-5x+25/4-33/4)
=-2(x-5/2)^2+33/2<=33/2
Dấu = xảy ra khi x=5/2
c: =x^2+x-2-2x^2-12x-18
=-x^2-11x-20
=-(x^2+11x+20)
=-(x^2+11x+121/4-41/4)
=-(x+11/2)^2+41/4<=41/4
Dấu = xảy ra khi x=-11/2