[toán 8] đại số

K

kool_boy_98

Giúp bạn nhá:

Bài 1:
$a) x^2-8x+19=x^2-2.x.4+16+3=(x-4)^2+3 \ge 3 > 0$ \forall $x$.
$b) 3x^2-6x+5=3x^2-6x+3+2=(\sqrt{3}x-\sqrt{3})^2+2 \ge 2 > 0$ \forall $x$.

Bài 2:
$a) A= x^2-4x+7=x^2-2.x.2+4+3=(x-2)^2+3$ \leq $3$ \forall $x$.
$b) B= 2x^2+4x+15=2x^2+4x+2+13=(\sqrt{2}x+\sqrt{2})^2+13$ \geq $13$ \forall $x$.
 
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