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\(a,\left(x-3\right)\left(x^2+3x+9\right)-\left(30+x^3\right)=x^3-27-30-x^3=-57\)

\(b,\left(2x-y\right)\left(4x^2+2xy+y^2\right)+y^3=8x^3-y^3+y^3=8x^3\)

\(a,\left(x+2\right)\left(x^2-2x+4\right)=x^3+2x\)

\(\Rightarrow\)\(x^3+8-x^3-2x=0\)

\(\Rightarrow\)\(2x=8\)

\(\Rightarrow\)\(x=4\)

\(b,\left(3x-2\right)\left(9x^2+6x+4\right)-27x^3=3x-5\)

\(\Rightarrow\)\(27x^3-8-27x^3-3x+5=0\)

\(\Rightarrow\)\(3x=-3\)

\(\Rightarrow\)\(x=-1\)

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

\(=2x^2+3x^3-3x-5x^2-5x\)

\(=3x^3-3x^2-8x\)

`2x^2 + 3x (x-1) (x+1) -5x(x+1)`

`= 2x^2 +3x (x^2-1) - 5x^2 -5x`

`= 2x^2 +3x^3 - 3x - 5x^2 - 5x`

`= 3x^3 - 3x^2 - 8x`

Đặt 

\(\Rightarrow\hept{\begin{cases}x=a-b\\y=a-c\\z=b-c\end{cases}}\)

Ta được

\(B=\frac{1}{axy}+\frac{1}{bxz}+\frac{1}{cyz}=\frac{bcz-acy+abx}{abcxyz}\)

\(=\frac{bc\left(b-c\right)-ac\left(a-c\right)+ab\left(a-b\right)}{abc\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{bc\left(b-c\right)-ac\left(a-b+b-c\right)+ab\left(a-b\right)}{abc\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{bc\left(b-c\right)-ac\left(a-b\right)-ac\left(b-c\right)+ab\left(a-b\right)}{abc\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{c\left(b-c\right)\left(b-a\right)+a\left(a-b\right)\left(b-c\right)}{abc\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{\left(b-c\right)\left(a-b\right)\left(a-c\right)}{abc\left(a-b\right)\left(a-c\right)\left(b-c\right)}\)

\(=\frac{1)}{abc}\)

Vậy ...

khó thế

Lê văn hải có oline thì tham khảo nha .

x^10 + x^5 + 1 
= x^10 + x^9 - x^9 + x^8 - x^8 + x^7 - x^7 + x^6 - x^6 + x^5 + x^5 - x^5 + x^4 - x^4 + x^3 - x^3 + x^2 - x^2 + x - x + 1 
= (x^10 + x^9 + x^8) - (x^9 + x^8 + x^7) + (x^7 + x^6 + x^5) - (x^6 + x^5 + x^4) + (x^5 + x^4 + x^3) - (x^3 + x^2 + x) + (x^2 + x + 1) 
= x^8 (x^2 + x + 1) - x^7 (x^2 + x + 1) + x^5 (x^2 + x + 1) - x^4 (x^2 + x + 1) + x^3 (x^2 + x + 1) - x (x^2 + x + 1) + (x^2 + x + 1) 
= (x^2 + x + 1) (x^8 - x^7 + x^5 - x^4 + x^3 - x + 1

mik xin chép lại cái đề bài

a) (x - 3)(x² +3x + 9)

=x² +3x² +9x -3x² - 9x -27

=x² - 27

b) (2y + z)(4y² -2yz + z²)

= 8y³ -4y²z + 2yz² + 4y²z -2yz² + z³

=8y³ + z³

mik lm hơi tắt chút nếu bn k hiểu thì bảo mik,mik lm chi tiết cho