Tìm gtnn lớp 8

X

xuanquynh97

a) $-x^2-8x+5=-(x^2+8x+16)+21$

$=-(x+4)^2+21$ \leq 21

Max=21 \Leftrightarrow $x=-4$

b) $5-x^2+2x-4x^2-4x$ (Bài này có sai đề không )

$=-5x^2-2x+5=-5(x^2+\dfrac{2}{5}x+\dfrac{1}{25})+\dfrac{26}{5}$

$=-5(x+\dfrac{1}{5})^2+\dfrac{26}{5}$ \leq $\dfrac{26}{5}$
 
X

xuanquynh97

c) $-4x^2-4x-2$ = $-(4x^2+4x+1)-1$

$=-(2x+1)^2-1$ \leq -1

Max=-1 \Leftrightarrow $x=\dfrac{-1}{2}$
 
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