Question

[Vũ Trần Giang]

a) \(a^2+b^2=1\)
Tìm min/max F = \(\dfrac{a}{b+2}\)
b)\(2a^2-2ab+5b^2=1\)
Tìm min/max G = \(\dfrac{\left(a+b\right)}{a-2b+2}\)

Answer 1

[Nguyễn Việt Lâm]

a.

\(F=\dfrac{a}{b+2}\Rightarrow F.b+2F=a\)

\(\Rightarrow2F=a-F.b\)

\(\Rightarrow4F^2=\left(a-F.b\right)^2\le\left(a^2+b^2\right)\left(1^2+F^2\right)=F^2+1\)

\(\Rightarrow3F^2\le1\)

\(\Rightarrow-\dfrac{1}{\sqrt{3}}\le F\le\dfrac{1}{\sqrt{3}}\)

Dấu "=" lần lượt xảy ra tại \(\left(a;b\right)=\left(-\dfrac{\sqrt{3}}{2};-\dfrac{1}{2}\right)\) và \(\left(\dfrac{\sqrt{3}}{2};-\dfrac{1}{2}\right)\)

b. Đặt \(\left\{{}\begin{matrix}a+b=x\\a-2b=y\end{matrix}\right.\) quay về câu a