Toán toán 9 violympic

L

leminhnghia1

Giải:

1, $d=AB+BC+CA$

2, $2x+xy=4 \iff 2x^2+x^2y=4x$

$\iff x^2y=2-2(x^2-2x+1)=2-2(x-1)^2 \le 2$

$Min=2$ Có đc khi: $x=1; y=2$

3, $x^2+xy+y^2-3(x+y)+3=(x+y-\dfrac{3}{2})^2+\dfrac{3}{4}(y-1)^2+\dfrac{15}{14} \ge \dfrac{15}{14}$
 
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