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\(A=x^2-3x+5\)
\(=x^2-3x+\frac{9}{4}+\frac{11}{4}\)
\(=\left(x-\frac{3}{2}\right)^2+\frac{11}{4}\)
\(\left(x-\frac{3}{2}\right)^2\ge0\Rightarrow A\ge\frac{11}{4}\)
Dấu "=" xảy ra khi \(x-\frac{3}{2}=0\Rightarrow x=\frac{3}{2}\)
Vậy Min A = \(\frac{11}{4}\Leftrightarrow x=\frac{3}{2}\)
a) \(A=x^2-3x+5\)
\("="\Leftrightarrow x=\frac{11}{4}\Rightarrow x=\frac{3}{2};\frac{11}{4}\)
b) \(B=\left(2x-1\right)^2+\left(x+2\right)^2\)
\("="\Leftrightarrow x=5\Rightarrow x=0;5\)
c) \(C=4x-x^2+3\)
\("="\Leftrightarrow x=7\Rightarrow x=2;7\)
d) \(D=x^4+x^2+2\)
\("="\Leftrightarrow x=2\Rightarrow x=0;2\)
\(A=x^2-6x+10=\left(x^2-6x+9\right)+1=\left(x-3\right)^2+1\ge1\forall x\)
Dấu "=" xảy ra <=> x = 3
Vậy MinA = 1
\(B=5x^2-10x+3=5\left(x^2-2x+1\right)-2=5\left(x-1\right)^2-2\ge-2\forall x\)
Dấu "=" xảy ra <=> x = 1
Vậy MinB = -2
\(C=2x^2+8x+y^2-10y+43=2\left(x^2+4x+4\right)+\left(y^2-10y+25\right)+10=2\left(x+2\right)^2+\left(y-5\right)^2+10\ge10\forall x,y\)
Dấu "=" xảy ra <=> x = -2 ; y = 5
Vậy MinC = 10
\(A=x^2-6x+10\)
\(=\left(x^2-6x+9\right)+1\)
\(=\left(x-3\right)^2+1\ge1\forall x\)
Dấu"=" xảy ra khi \(x-3=0\Leftrightarrow x=3\)
Vậy \(Min_A=1\Leftrightarrow x=3\)
b,\(B=5x^2-10x+3\)
\(=5\left(x^2-2x+1\right)-2\)
\(=5\left(x-1\right)^2-2\ge-2\forall x\)
Dấu"=" xảy ra khi \(x-1=0\Leftrightarrow x=1\)
Vậy \(Min_B=-2\Leftrightarrow x=1\)
c,\(C=2x^3+8x+y^2-10+43\)
\(=2x^2+8x+8+y^2-10y+25+10\)
\(=2\left(x^2+4x+4\right)+\left(y^2-10y+25\right)+10\)
\(=2\left(x+2\right)^2+\left(y-5\right)^2+10\ge10\forall x,y\)
Dấu"=" xảy ra khi \(\orbr{\begin{cases}x+2=0\\y-5=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=-2\\y=5\end{cases}}}\)
Vậy \(Min_C=10\Leftrightarrow x=-2;y=5\)
mk gợi ý, phần còn lại tự làm
a) \(A=x^2+2x+5=\left(x+1\right)^2+4\ge4\)
b) \(B=4x^2+4x+11=\left(2x+1\right)^2+10\ge10\)
c) \(\left(x-1\right)\left(x+2\right)\left(x+3\right)\left(x+6\right)=\left(x^2+5x-6\right)\left(x^2+5x+6\right)\)
\(=\left(x^2+5x\right)^2-36\ge-36\)
d) \(D=x^2-2x+y^2-4y+7=\left(x-1\right)^2+\left(y-2\right)^2+2\ge2\)
e) \(E=x^2-4xy+5y^2+10x-22y+28=\left(x-2y+5\right)^2+\left(y-1\right)^2+2\ge2\)
a) A = x2 + 2x + 5
= x2 + 2x + 1 + 4
= ( x + 1 )2 + 4
Nhận xét :
( x + 1 )2 > 0 với mọi x
=> ( x + 1 )2 + 4 > 4
=> A > 4
=> A min = 4
Dấu " = " xảy ra khi : ( x + 1 )2 = 0
=> x + 1 = 0
=> x = - 1
Vậy A min = 4 khi x = - 1
b) B = 4x2 + 4x + 11
= ( 2x )2 + 4x + 1 + 10
= ( 2x + 1 )2 + 10
Nhận xét :
( 2x + 1 )2 > 0 với mọi x
=> ( 2x + 1 )2 + 10 > 10
=> B > 10
=> B min = 10
Dấu " = " xảy ra khi : ( 2x + 1 )2 = 0
=> 2x + 1 = 0
=> x = \(\frac{-1}{2}\)
Vậy Bmin = 10 khi x = \(\frac{-1}{2}\)
c) C = ( x - 1 ) ( x + 2 ) ( x + 3 ) ( x + 6 )
= [ ( x - 1 ) ( x + 6 ) ] [ ( x + 2 ) ( x + 3 ) ]
= ( x2 + 5x - 6 ) ( x2 + 5x + 6 )
= ( x2 + 5x ) 2 - 62
= ( x2 + 5x )2 - 36
Nhận xét :
( x2 + 5x )2 > 0 với mọi x
=> ( x2 + 5x )2 - 36 > - 36
=> C > - 36
=> C min = - 36
Dấu " = " xảy ra khi : ( x2 + 5x )2 = 0
=> x2 + 5x = 0
=> x ( x + 5 ) = 0
=> \(\orbr{\begin{cases}x=0\\x+5=0\end{cases}}\)
=> \(\orbr{\begin{cases}x=0\\x=-5\end{cases}}\)
Vậy C min = - 36 khi x = 0 hoặc x = - 5
d) D = x2 - 2x + y2 - 4y + 7
= ( x2 - 2x + 1 ) + ( y2 - 4x + 4 ) + 2
= ( x - 1 )2 + ( y - 2 )2 + 2
Nhận xét :
( x - 1 )2 > 0 với mọi x
( y - 2 )2 > 0 với mọi y
=> ( x - 1 )2 + ( y - 2 )2 > 0
=> ( x - 1 )2 + ( y - 2 )2 + 2 > 2
=> D > 2
=> D min = 2
Dấu " = " xảy ra khi : \(\hept{\begin{cases}\left(x-1\right)^2=0\\\left(y-2\right)^2=0\end{cases}}\)
=> \(\hept{\begin{cases}x-1=0\\y-2=0\end{cases}}\)
=> \(\hept{\begin{cases}x=1\\y=2\end{cases}}\)
Vậy D min = 2 khi x = 1 và y = 2
\(1,a,A=x^2-6x+25\)
\(=x^2-2.x.3+9-9+25\)
\(=\left(x-3\right)^2+16\)
Ta có :
\(\left(x-3\right)^2\ge0\)Với mọi x
\(\Rightarrow\left(x-3\right)^2+16\ge16\)
Hay \(A\ge16\)
\(\Rightarrow A_{min}=16\)
\(\Leftrightarrow x=3\)
a: \(=-\left(x^2+10x-11\right)\)
\(=-\left(x^2+10x+25-36\right)\)
\(=-\left(x+5\right)^2+36< =36\)
Dấu '=' xảy ra khi x=-5
b: \(=-\left(x^2-6x+5\right)\)
\(=-\left(x^2-6x+9-4\right)\)
\(=-\left(x-3\right)^2+4< =4\)
Dấu '=' xảy ra khi x=3
c: \(=-2\left(x^2-x+\dfrac{5}{2}\right)\)
\(=-2\left(x^2-x+\dfrac{1}{4}+\dfrac{9}{4}\right)\)
\(=-2\left(x-\dfrac{1}{2}\right)^2-\dfrac{9}{2}< =-\dfrac{9}{2}\)
Dấu '=' xảy ra khi x=1/2
d: \(=2x+8-x^2-4x\)
\(=-x^2-2x+8\)
\(=-\left(x^2+2x-8\right)\)
\(=-\left(x^2+2x+1-9\right)\)
\(=-\left(x+1\right)^2+9< =9\)
Dấu '=' xảy ra khi x=-1
a, x2 + 10x + 27
Đặt A = x2 + 2. x. 5 + 52 + 2
= ( x + 5 )2 + 2
Vì ( x + 5 )2 \(\ge\)0 với mọi x
=> ( x + 5 )2 + 2 \(\ge\)2 với mọi x
Hay A \(\ge\)2
Dấu " = " xảy ra khi:
( x + 5 )2 = 0
x + 5 = 0
x = - 5
Vậy Min A = 2 khi x = - 5
b, x2 + x + 7
Đặt B = x2 + x + 7
\(=x^2+x+\frac{1}{4}+\frac{27}{4}\)
\(=\left[x^2+2\cdot x\cdot\frac{1}{2}+\left(\frac{1}{2}\right)^2\right]+\frac{27}{4}\)
\(=\left(x+\frac{1}{2}\right)^2+\frac{27}{4}\)
Vì \(\left(x+\frac{1}{2}\right)^2\ge0\)với mọi x
\(\Rightarrow\left(x+\frac{1}{2}\right)^2+\frac{27}{4}\ge\frac{27}{4}\)với mọi x
Hay B \(\ge\frac{27}{4}\)
Dấu " = " xảy ra khi:
\(\left(x+\frac{1}{2}\right)^2=0\)
\(x+\frac{1}{2}=0\)
\(x=-\frac{1}{2}\)
Vậy Min B = \(\frac{27}{4}\)khi x = \(-\frac{1}{2}\)
a) x2 + 10 x + 27 =( x2 + 2. 5 . x + 52 ) + 2 = ( x + 5 ) 2 + 2
Vì ( x + 5 ) 2 \(\ge\) 0 với mọi x nên ( x + 5 ) 2 + 2 \(\ge\) 2 với mọi x
Dấu bằng xảy ra \(\Leftrightarrow\)x + 5 = 0 \(\Leftrightarrow\) x = -5
b) x2 + x + 7 = 0 \(\Leftrightarrow\) x2 + 2. x . \(\frac{1}{2}\)+ \(\left(\frac{1}{2}\right)^2\) + \(\frac{27}{4}\) = 0 \(\Leftrightarrow\)( x + 1/2) 2 + 27/4 = 0
Vì ( x + 1/2 )2 \(\ge\) 0 với mọi x nên ( x + 1/2) 2 + 27/4 \(\ge\)27/4 với mọi x
Dấu bằng xảy ra \(\Leftrightarrow\)x+ 1/2 = 0 \(\Leftrightarrow\) x = ---\(\frac{1}{2}\)
c + d ) Tương tự a, b
e) x2 + 14 x + y2 - 2y +7 = 0 \(\Leftrightarrow\) ( x2 + 2. x. 7 + 72 ) + ( y2 -- 2y + 1 ) -43 = 0 \(\Leftrightarrow\) ( x + 7 ) 2 + ( y -- 1 ) 2 --43 = 0 ( 1 )
Vì ( x + 7 )2 \(\ge\) 0 và ( y -- 1 )2 \(\ge\) 0 với mọi x, y nên ( 1 ) \(\ge\) --43 với mọi x, y
Dấu bằng xảy ra \(\Leftrightarrow\) \(\hept{\begin{cases}x+7=0\\y-1=0\end{cases}}\) \(\Leftrightarrow\) \(\hept{\begin{cases}x=-7\\y=1\end{cases}}\)
\(A=x^2-10x+30=x^2-10x+25+5=\left(x-5\right)^2+5\ge5\)
Vậy GTNN của A là 5 khi x = 5
\(B=4x^2+4x+9=4x^2+4x+1+8=\left(2x+1\right)^2+8\ge8\)
Vậy GTNN của B là 8 khi x = \(-\dfrac{1}{2}\)
\(C=9x^2-12x+20=9x^2-12+4+16=\left(3x-2\right)^2+16\ge16\)
Vậy GTNN của C là 16 khi x = \(\dfrac{2}{3}\)
\(D=x^2+x+1=x^2+x+\dfrac{1}{4}+\dfrac{3}{4}=\left(x+\dfrac{1}{2}\right)^2+\dfrac{3}{4}\ge\dfrac{3}{4}\)
Vậy GTNN của D là \(\dfrac{3}{4}\) khi x = \(-\dfrac{1}{2}\)
\(E=2x^2+3x+5=2\left(x^2+\dfrac{3}{2}x+\dfrac{9}{16}\right)+\dfrac{31}{8}=2\left(x+\dfrac{3}{4}\right)^2+\dfrac{31}{8}\ge\dfrac{31}{8}\)
Vậy GTNN của E là \(\dfrac{31}{8}\) khi x = \(-\dfrac{3}{4}\)
\(F=3x^2-7x+6=3\left(x^2-\dfrac{7}{3}x+\dfrac{49}{36}\right)+\dfrac{23}{12}=\left(x-\dfrac{7}{6}\right)^2\ge\dfrac{23}{12}\)Vậy GTNN của F là \(\dfrac{23}{12}\) khi x = \(\dfrac{7}{6}\)
+) \(A=x^2+2x-9=x^2+2x+1-10=\left(x+1\right)^2-10\ge-10\)
Min A = -10 \(\Leftrightarrow x=-1\)
+) \(B=x^2+5x-1=x^2+5x+\frac{25}{4}-\frac{29}{4}=\left(x+\frac{5}{2}\right)^2-\frac{29}{4}\ge\frac{-29}{4}\)
Min B = -29/4 \(\Leftrightarrow x=\frac{-5}{2}\)
+) \(C=x^2+4x=x^2+4x+4-4=\left(x+2\right)^2-4\ge-4\)
Min C = -4 \(\Leftrightarrow x=-2\)
+) \(D=x^2-8x+17=x^2-8x+16+1=\left(x-4\right)^2+1\ge1\)
Min D = 1 \(\Leftrightarrow x=4\)
+) \(E=x^2-7x+1=x^2-7x+\frac{49}{4}-\frac{45}{4}=\left(x-\frac{7}{2}\right)-\frac{45}{4}\ge-\frac{45}{4}\)
Min E = -45/4 \(\Leftrightarrow x=\frac{7}{2}\)
A = x2 + 2x - 9
= ( x2 + 2x + 1 ) - 10
= ( x + 1 )2 - 10 ≥ -10 ∀ x
Đẳng thức xảy ra <=> x + 1 = 0 => x = -1
=> MinA = -10 <=> x = -1
B = x2 + 5x - 1
= ( x2 + 5x + 25/4 ) - 29/4
= ( x + 5/2 )2 - 29/4 ≥ -29/4 ∀ x
Đẳng thức xảy ra <=> x + 5/2 = 0 => x = -5/2
=> MinB = -29/4 <=> x = -5/2
C = x2 + 4x
= ( x2 + 4x + 4 ) - 4
= ( x + 2 )2 - 4 ≥ -4 ∀ x
Đẳng thức xảy ra <=> x + 2 = 0 => x = -2
=> MinC = -4 <=> x = -2
D = x2 - 8x + 17
= ( x2 - 8x + 16 ) + 1
= ( x - 4 )2 + 1 ≥ 1 ∀ x
Đẳng thức xảy ra <=> x - 4 = 0 => x = 4
=> MinD = 1 <=> x = 4
E = x2 - 7x + 1
= ( x2 - 7x + 49/4 ) - 45/4
= ( x - 7/2 )2 - 45/4 ≥ -45/4 ∀ x
Đẳng thức xảy ra <=> x - 7/2 = 0 => x = 7/2
=> MinE = -45/4 <=> x = 7/2
a,\(x^2-6x-17=x^2-2\cdot3x+9-26=\left(x-3\right)^2-26\ge-26\)
b, \(x^2-10x=x^2-2\cdot5x+25-25=\left(x-5\right)^2-25\ge-25\)
c,\(3x^2-12x+5=3x^2-2\cdot\sqrt{3}x\cdot2\sqrt{3}+12-7=\left(\sqrt{3}x-2\sqrt{3}\right)^2-7\ge-7\)
d,\(2x^2-x-1=2x^2-2\cdot\sqrt{2}x\cdot\dfrac{1}{2\sqrt{2}}+\dfrac{1}{8}-\dfrac{9}{8}=\left(\sqrt{2}x-\dfrac{1}{2\sqrt{2}}\right)^2-\dfrac{9}{8}\ge-\dfrac{9}{8}\)
e,\(x^2+y^2-8x+4y+27=x^2-2\cdot4x+16+y^2+2\cdot2y+4+7=\left(x-4\right)^2+\left(y+2\right)^2+7\ge7\)
f,\(x\left(x-6\right)=x^2-6x=x^2-2\cdot3x+9-9=\left(x-3\right)^2-9\ge-9\)
h,\(\left(x-2\right)\cdot\left(x-5\right)\cdot\left(x^2-7x-10\right)=\left(x^2-7x+10\right)\left(x^2-7x-10\right)=\left(x^2-7x\right)^2-100\ge-100\)
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