= 8x^3 -27 -8x^3 +8x

= -27+8x

a)  \(A=4x^2+8x+7=\left(2x+2\right)^2+3\ge3\)

Vậy Min \(A=3\) khi \(x=-1\)

b)  \(B=x^2-2x+5=\left(x-1\right)^2+4\ge4\)

Vậy Min \(B=4\)khi   \(x=1\)

c)  \(C=x^2+4x+10=\left(x+2\right)^2+6\ge6\)

Vậy Min \(C=6\)khi  \(x=-2\)

thank bn nha

\(\frac{1}{27}+x^3\)

\(=\left(\frac{1}{3}\right)^3+x^3\)

\(=\left(\frac{1}{3}+x\right)\left(\frac{1}{9}-\frac{1}{3}x+x^2\right)\)

\(\frac{1}{27}+x^3=\left(\frac{1}{3}\right)^3+x^3\)\(\left(\frac{1}{3}+x\right)\left[\left(\frac{1}{3}\right)^2-\frac{1}{3}x+x^2\right]\)

\(x^2-6x+10\)

\(=\left(x^2-6x+9\right)+1\)

\(=\left(x-3\right)^2+1>0\)  mọi x

p/s: chúc bạn hk tốt

\(x^2-6x+10\)

\(=\left(x^2-6x+9\right)+1\)

\(=\left(x-3\right)+1>0\)

Code : Breacker

\(^{x^2\left(9-15x^2\right)+3x\left(7+5x^2\right)=1}\)

\(9x^2-15x^4+21x+15x^3-1=0\)

\(\left(3x\right)^2-1^2-15x^4+21x+15x^3=0\)

\(\left(3x-1\right)\left(3x+1\right)5x\left(-x^3+7+5x^2\right)=0\)

\(TH1:3x-1=0\\ 3x=1\\ x=\frac{1}{3}\)            \(TH2:3x+1=0\\ 3x=-1\\ x=\frac{-1}{3}\)     \(TH3:5x=0\\ x=0\)    

 \(TH4:-x^3+7+5x^2=0\\ x^2\left(5-x\right)=7\)(loại)

Vậy x thuộc{1/3;-1/3;0}

mik ko bít

I don't now

................................

.............

\(x^4+81\)

\(=x^4+3^4\)

\(=\left(x^2+3^2\right)^2-2x^23^2\)

\(=\left(x^2+\sqrt{2}x3+3^2\right)\left(x^2-\sqrt{2}x3+3^2\right)\)

nguồn gg

\(x^4+81\)

\(=x^4+18x^2+81-18x^2\)

\(=\left(x^2+9\right)^2-18x^2\)

\(=\left(x^2-3\sqrt{2}x+9\right)\left(x^2+3\sqrt{2}x+9\right)\)