Tìm gtln, gtnn của |3x+7|+13/2.|3x+7|+6
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Bài 2:
a: Ta có: \(x^2+4x+7\)
\(=x^2+4x+4+3\)
\(=\left(x+2\right)^2+3\ge3\forall x\)
Dấu '=' xảy ra khi x=-2
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a,Ta có :\(A=x\left(x-6\right)=x^2-6x\)
\(=x^2-6x+9-9\)
\(=\left(x-3\right)^2-9\)
Vì: \(\left(x-3\right)^2\ge0\forall x\)
\(\Rightarrow\)\(\left(x-3\right)^2-9\ge-9\forall x\)
Hay: \(A\ge-9\forall x\)
Dấu = xảy ra khi (x-3)^2=0
<=>x=3
Vậy Min A= -9 tại x=3
b,Ta có: \(B=-3x\left(x+3\right)-7\)
\(=-3x^2-9x-7\)
\(=-3\left(x^2+3x+\frac{7}{3}\right)\)
\(=-3\left[\left(x^2+3x+\frac{9}{4}\right)+\frac{1}{12}\right]\)
\(=-3\left[\left(x+\frac{3}{2}\right)^2+\frac{1}{12}\right]\)
\(=-3\left(x+\frac{3}{2}\right)^2-\frac{1}{4}\)
Vì: \(-3\left(x+\frac{3}{2}\right)^2\le0\forall x\)
\(\Rightarrow-3\left(x+\frac{3}{2}\right)^2-\frac{1}{4}\le\frac{-1}{4}\forall x\)
Hay \(B\le\frac{-1}{4}\forall x\)
Dấu = xảy ra khi \(-3\left(x+\frac{3}{2}\right)^2=0\)
\(\Rightarrow x=\frac{-3}{2}\)
Vậy Max B=-1/4 tại x=-3/2
a) \(A=x\left(x-6\right)=x^2-6x+9-9=\left(x-3\right)^2-9\ge-9\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(x=3\)
Vậy Min A = -9 khi x = 3
b) \(B=-3x\left(x+3\right)-7=-3x^2-9x-7=-3\left(x^2+9x+20,25\right)+53,75\)
\(=-3\left(x+4,5\right)^2+53,75\le53,75\)
Dấu "=" xảy ra \(\Leftrightarrow\)\(x=-4,5\)
Vậy Max B = 53,75 khi x = -4,5
![](https://rs.olm.vn/images/avt/0.png?1311)
|3x-7|+|3x-2|+8 >= 5+8 = 13
Dấu "=" xảy ra <=> 3/2 <= x <= 7/3
k mk nha
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a)A=4(x+11/8)^2 -153/16
Min A=-153/16 khi x=-11/8
b)B=3(x-1/3)^2 -4/3
Min B=-4/3 khi x=1/3
Bài 1:
a) \(A=4x^2+11x-2=\left(4x^2+11x+\dfrac{121}{16}\right)-\dfrac{153}{16}=\left(2x+\dfrac{11}{4}\right)^2-\dfrac{153}{16}\ge-\dfrac{153}{16}\)
\(minA=-\dfrac{153}{16}\Leftrightarrow x=-\dfrac{11}{8}\)
b) \(B=3x^2-2x-1=3\left(x^2-\dfrac{2}{3}x+\dfrac{1}{9}\right)-\dfrac{4}{3}=3\left(x-\dfrac{1}{3}\right)^2-\dfrac{4}{3}\ge-\dfrac{4}{3}\)
\(minB=-\dfrac{4}{3}\Leftrightarrow x=\dfrac{1}{3}\)
Bài 2:
a) \(A=-x^2+3x-1=-\left(x^2-3x+\dfrac{9}{4}\right)+\dfrac{5}{4}=-\left(x-\dfrac{3}{2}\right)^2+\dfrac{5}{4}\le\dfrac{5}{4}\)
\(maxA=\dfrac{5}{4}\Leftrightarrow x=\dfrac{3}{2}\)
b) \(B=-x^2-4x+7=-\left(x^2+4x+4\right)+11=-\left(x+2\right)^2+11\le11\)
\(maxB=11\Leftrightarrow x=-2\)
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1) \(A=\frac{2018x^2-2.2018x+2018^2}{2018x^2}=\frac{\left(x-2018\right)^2+2017x^2}{2018x^2}=\frac{\left(x-2018\right)^2}{2018x^2}+\frac{2017}{2018}\)
vì \(\frac{\left(x-2018\right)^2}{2018x^2}\ge0\Rightarrow\frac{\left(x-2018\right)^2}{2018x^2}+\frac{2017}{2018}\ge\frac{2017}{2018}\)
dấu = xảy ra khi x-2018=0
=> x=2018
Vậy Min A=\(\frac{2017}{2017}\)khi x=2018
2) \(B=\frac{3x^2+9x+17}{3x^2+9x+7}=\frac{3x^2+9x+7+10}{3x^2+9x+7}=1+\frac{10}{3x^2+9x+7}=1+\frac{10}{3.x^2+9x+7}\)
\(=1+\frac{10}{3.\left(x^2+9x\right)+7}=1+\frac{10}{3.\left[x^2+\frac{2.x.3}{2}+\left(\frac{3}{2}\right)^2\right]-\frac{9}{4}+7}=1+\frac{10}{3.\left(x+\frac{9}{2}\right)^2+\frac{1}{4}}\)
để B lớn nhất => \(3.\left(x+\frac{3}{2}\right)^2+\frac{1}{4}\)nhỏ nhất
mà \(3.\left(x+\frac{3}{2}\right)^2+\frac{1}{4}\ge\frac{1}{4}\)vì \(3.\left(x+\frac{3}{2}\right)^2\ge0\)
dấu = xảy ra khi \(x+\frac{3}{2}=0\)
=> x=\(-\frac{3}{2}\)
Vậy maxB=\(41\)khi x=\(-\frac{3}{2}\)
3) \(M=\frac{3x^2+14}{x^2+4}=\frac{3.\left(x^2+4\right)+2}{x^2+4}=3+\frac{2}{x^2+4}\)
để M lớn nhất => x2+4 nhỏ nhất
mà \(x^2+4\ge4\)(vì x2 lớn hơn hoặc bằng 0)
dấu = xảy ra khi x2 =0
=> x=0
Vậy Max M\(=\frac{7}{2}\)khi x=0
ps: bài này khá dài, sai sót bỏ qua =))
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a) A = x( 5 - 3x ) = -3x2 + 5x = -3( x2 - 5/3x + 25/36 ) + 25/12
= -3( x - 5/6 )2 + 25/12 ≤ +25/12 ∀ x
Dấu "=" xảy ra khi x = 5/6
Vậy MaxA = 25/12 <=> x = 5/6
b) Từ x + y = 7 => x = 7 - y
Ta có : xy = ( 7 - y ).y = 7y - y2 = -( y2 - 7y + 49/4 ) + 49/4 = -( y - 7/2 )2 + 49/4 ≤ 49/4 ∀ y
Dấu "=" xảy ra <=> y = 7/2 => x = 7/2
Vậy Max(xy) = 49/4 <=> x = y = 7/2
( nếu cho x,y dương thì Cauchy nhanh gọn luôn :)) )
\(A=\left|3x+7\right|+\frac{13}{2}\left|3x+7\right|+6\)
Có: \(\left|3x+7\right|\ge0;\frac{13}{2}\left|3x+7\right|\ge0\)
\(\Rightarrow\left|3x+7\right|+\frac{13}{2}\left|3x+7\right|+6\ge6\)
Dấu '=' xảy ra khi: \(\left|3x+7\right|+\frac{13}{2}\left|3x+7\right|=0\)
\(\Leftrightarrow\left|3x+7\right|.\left(\frac{13}{2}+1\right)=0\)
\(\Leftrightarrow\left|3x+7\right|=0\Leftrightarrow3x+7=0\)
\(\Leftrightarrow x=-\frac{7}{3}\)
Vậy: \(Min_A=6\) tại \(x=-\frac{7}{3}\)