Question

Tìm GTNN của

a/ A=\(\dfrac{x^2-x+3}{\sqrt{1-x^3}}\) với x<1

b/ B= \(\sqrt{-x^2+4x+21}-\sqrt{-x^2+3x+10}với-2\le x\le5\)

Answer 1

Ta có: \(\left(-x^2+4x+21\right)-\left(-x^2+3x+10\right)=x+11>0\Rightarrow B>0\)

\(B^2=\left(x+3\right)\left(7-x\right)+\left(x+2\right)\left(5-x\right)-2\sqrt{\left(x+3\right)\left(7-x\right)\left(x+2\right)\left(5-x\right)}=\left(\sqrt{\left(x+3\right)\left(5-x\right)}-\sqrt{\left(x+2\right)\left(7-x\right)}\right)^2+2\ge2\)

\(\Rightarrow B\ge\sqrt{2}\)

GTNN của B là \(\sqrt{2}\Leftrightarrow x=\dfrac{1}{3}\)