a)A=[TEX]2x^2[/TEX]+5x+8
b)B=(x-2)(x-5)([TEX]x^2[/TEX]-7x-10)
bai 2:Ti`m x(3x-1)(x+5)-[TEX](x+2)^2[/TEX]=-9
bài 1: a)A=[TEX]2x^2[/TEX]+5x+8:khi (35):
= 2.( x^2 + 5x/2 +4)
= 2.( x^2 + 5x/2 + 25/16 - 25/16 + 4)
= 2. [ ( x + 5/4)^2 + 39/16 ]
= 2. ( x + 5/4)^2 + 39/8
:khi (143):Vì ( x + 5/4)^2 \geq 0 mọi x
nên 2. ( x + 5/4)^2 \geq 0 mọi x
\Rightarrow 2. ( x + 5/4)^2 + 39/8 \geq 39/8 mọi x
\Rightarrow A
min = 39/8 \Leftrightarrow 2. ( x + 5/4)^2 =0
\Rightarrow
x + 5/4 =0
\Leftrightarrow
x = -5/4
Vậy..............:khi (196): câu tự suy ra nhé
b)B=(x-2)(x-5)([TEX]x^2[/TEX]-7x-10)
= ( x^2 - 7x + 10).( x^2-7x -10)
= ( x^2 - 7x )^2 - 10^2
=( x^2 - 7x )^2 - 100
:khi (90):Vì ( x^2 - 7x )^2 \geq0 mọi x
\Rightarrow ( x^2 - 7x )^2 - 100 \geq -100 mọi x
\Rightarrow B min = -100 \Leftrightarrow ( x^2 - 7x )^2 = 0
\Leftrightarrow x.( x -7 ) =0
\Leftrightarrow [TEX]\left[\begin{x =0}\\{ x -7 = 0} [/TEX]
\Leftrightarrow [TEX]\left[\begin{x=0}\\{x = 7} [/TEX]
Vậy..............:khi (143):
bai 2:Ti`m x(3x-1)(x+5)-[TEX](x+2)^2[/TEX]=-9[/QUOTE]
\Leftrightarrow x.( 3x^2 + 14x - 5) - x^2 - 4x -4 = -9
\Leftrightarrow 3x^3 +14x^2 -5x - x^2 - 4x -4 +9 =0
\Leftrightarrow 3x^3 + 13x^2 -9x +5 =0
\Leftrightarrow 3x^3 +15x^2 - 2x^2 - 10x + x +5 =0:khi (3):
\Leftrightarrow 3x^2.( x +5) - 2x.( x +5) + ( x +5) =0
\Leftrightarrow ( 3x^2 - 2x +1).( x +5) = 0
Vì 3x^2 - 2x +1 > 0 mọi x...............cái nèy tự chứng minh nha!!
Nên ( 3x^2 - 2x +1).( x +5) = 0
\Leftrightarrow x +5 =0
\Leftrightarrow x = -5 :khi (57):
[TẶNG BẠN] TRỌN BỘ Bí kíp học tốt 08 môn
