`A=5x^2-3x+2`
`=5x^2-3x+\frac{9}{20}+\frac{31}{20}`
`=((\sqrt{5}x)^2+2.\frac{3}{2}x+(\frac{3}{2\sqrt{5}})^2)+\frac{31}{20}`
`=(\sqrt{5}x+\frac{3}{2\sqrt{5}))^2+\frac{31}{20}>=\frac{31}{20}`
Dấu bằng xảy ra khi
`\sqrt{5}x+\frac{3}{2\sqrt{5}}=0`
`\sqrt{5}x=\frac{-3}{2\sqrt{5}}`
`=>2.5x=-3`
`=>10x=-3`
`=>x=-0,3`
`=>x=\frac{-3}{10}`
Vậy `A` min `=\frac{31}{20}` khi `x=\frac{-3}{10}`
`D=7x^2+3x+1`
`=7x^2+3x+\frac{9}{28}+\frac{19}{28}`
`=((\sqrt{7}x)^2 +2.\frac{3}{2}+(\frac{3}{2\sqrt{7}})^2)+\frac{19}{28}`
`=(\sqrt{7}x+\frac{3}{2\sqrt{7}))^2+\frac{19}{28}>=\frac{19}{28}`
Dấu bằng xảy ra khi
`\sqrt{7}x+\frac{3}{2\sqrt{7}}=0`
`=>\sqrt{7x}=\frac{-3}{2\sqrt{7}}=0`
`=>2.7x=-3`
`=>x=\frac{-3}{14}`
Vậy `D` min`=\frac{19}{28}` khi `x=\frac{-3}{14}`
