a: =a+b-a+b=2a

b: =xy+yz-x+x+xy-2yz=-yz

c: =2m+7n-9mn-6mn-15m+27n+21

=-13m+34n-15mn+21

\(\left(a+b\right)-\left(a-b\right).\\ =a+b-a+b.\\ =2b.\\ \left(xy+yz-x\right)+\left(x+xy-2yz\right).\\ =xy+yz-x+x+xy-2yz.\\ =2xy-yz.\\ =\left(2m+7n-9mn\right)-\left(6mn+15m-27n-21\right).\\ =2m+7n-9mn-6mn-15m+27n+21.\\ =-13m+34n-15mn+21.\)

a) Rút gọn được A = ( k 3  – 64) – (128 +  k 3 ) = -192.

b) Rút gọn được B = -19 m 3  + 35 n 3 .

\(\lim\limits_{ }\sqrt[3]{27n^3-7n^2}-3n=\lim\limits\dfrac{\left(27n^3-7n^2\right)-27n^3}{\left(\sqrt[3]{27n^3-7n^2}\right)^2+\sqrt[3]{27n^3-7n^2}.3n+9n^2}_{ }=\)

\(\lim\limits_{ }\dfrac{-7n^2}{n^2\left[\left(\sqrt[3]{27-\dfrac{7}{n}}\right)^2+\sqrt[3]{27-\dfrac{7}{n}}.3+9\right]}=\dfrac{-7}{27}\)

Nhờ các cao thủ giải dùm mình cái nha. Tks.

Tìm Lim ( )

Bài 1: 

a: Ta có: \(A=\left(k-4\right)\left(k^2+4k+16\right)-\left(k^3+128\right)\)

\(=k^3-64-k^3-128\)

=-192

b: Ta có: \(B=\left(2m+3n\right)\left(4m^2-6mn+9n^2\right)-\left(3m-2n\right)\left(9m^2+6mn+4n^2\right)\)

\(=8m^3+27n^3-27m^3+8n^3\)

\(=-19m^3+35n^3\)

Bài 4: 

a: Ta có: \(\left(x-1\right)^3+\left(2-x\right)\left(4+2x+x^2\right)+3x\left(x+2\right)=16\)

\(\Leftrightarrow x^3-3x^2+3x-1+8-x^3+3x^2+6x=16\)

\(\Leftrightarrow9x=9\)

hay x=1

b: ta có: \(\left(x+2\right)\left(x^2-2x+4\right)-x\left(x^2-2\right)=15\)

\(\Leftrightarrow x^3+8-x^3+2x=15\)

\(\Leftrightarrow2x=7\)

hay \(x=\dfrac{7}{2}\)

Đề bài là j vậy bạn???

2m + 7n + mn

= 2 . ( - 7 ) + 7 . ( - 5 ) + - 7 . ( - 5 )
= - 14 + ( - 35 ) + 35

= - 14

-14 đúng 100%

2m + 7n + mn

= 2 . ( - 7 ) + 7 . ( - 5 ) + - 7 . ( - 5 )

= - 14 + ( - 35 ) + 35

= - 14