`#28052008`
`B=x^2+10x+30`
`B=x^2+10x+25+5`
`B=(x^2+10x+25)+5`
`B=[(x)^2+2*5*x+5^2]+5`
`B=(x+5)^2+5`
Ta có: `(x+5)^2 >= 0 ∀ x`
`⇔(x+5)^2+5 >= 5`
Vậy bt luôn dương
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`C=9x^2+4x+1`
`B=9x^2+4x+4/9-4/9+1`
`B=(x^2+4x+4/9)+5/9`
`B=[(3x)^2+2*3*x*2/3+(2/3)^2]+5/9`
`B=(3x+2/3)^2+5/9`
Ta có: `(3x+5/9)^2 >= 0 ∀ x`
`⇔(3x+2/3)^2+5/9 >= 5/9`
Vậy bt luôn dương
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`B=4x^2-12x+15`
`B=4x^2-12x+9+6`
`B=(4x^2-12x+9)+6`
`B=[(2x)^2+2*3*2*x+3^2]+6`
`B=(2x+3)^2+6`
Ta có: `(2x+3)^2 >= 0 ∀ x`
`⇔(2x+3)^2+6 >= 6`
Vậy bt luôn dương
