a)

=25

b)

=1

HT

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

\(\Rightarrow49-48=1\)

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

\(=3\left[\left(x+y\right)^2-2xy\right]-2\left(x+y\right)\left[\left(x+y\right)^2-3xy\right]\)

Thiếu xy nhé 

a, \(A=\left(x-1\right)\left(x^2+x+1\right)-\left(x+1\right)\left(x^2-x+1\right)=x^3-1-x^3-1=-2\)

b, \(B=2x\left(4x+1\right)-8x^2\left(x+1\right)+\left(2x\right)^3-2x+3\)

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

c, \(C=\left(x-1\right)^3+\left(x+1\right)^3+2x\left(x+2\right)\left(x-2\right)\)

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

\(=8x^3-6x\left(x^2-1\right)+2x^3-8x=8x^3-6x^3+6x+2x^3-8x=4x^3-2\)

d, \(D=\left(x+y-5\right)^2-2\left(x+y-5\right)\left(x+3\right)+x^2+6x+9\)

\(=\left(x+y-5-x-3\right)^2=\left(y-8\right)^2=y^2-16x+64\)

chắc...đúng

Tam giác đều không có tâm đối xứng.

5 . chu vi hình thang là 4x7=28 (ABG)

6.(25+10) x24:2 = 79 

7 a hình thang

b . chu vi tứ giác là 15x9=135 

(20^2+18^2+16^2+.........+4^2+2^2)-(19^2+17^2+15^2+.........+3^2+1^2)

=(20+18+16+..+4+2)^2-(19+17+15+...+3+1)^2

=100^2-100^2

=0

https://h7.net/hoi-dap/toan-8/tinh-nhanh-20-2-18-2-16-2-4-2-2-2-19-2-17-2--faq432725.html

Bạn copy link ,ở đây là câu trả lời nhé

\(d,\left(x-20\right)\left(x^2+2x+4\right)-x^2\left(x+1\right)=-17\)

\(x^3+2x^2+4x-20x^2-40x-80-x^3-x^2=-17\)

\(-19x^2-36x-63=0\)

\(\Delta=\left(-36\right)^2-\left(-63.-19.4\right)=-3492< 0\)

pt vô nghiệm

Trả lời:

( x - 3 ) ( x² + 3x + 9 ) - ( x² - 27x )

= x³ - 27 - x² + 27x

= x³ - x² + 27x - 27

  ( 3 ----- 3)-----no biết       
 

1.\(13.87+13.12+13\)

\(=13\left(87+12+1\right)\)

\(=13.100=1300\)

2.Đề sai à ???

3.\(x\left(x+4\right)-x\left(x-6\right)\)

\(=x^2+4x-x^2+6x\)

\(=10x\)

\(=10.123=1230\)

1, \(13.87+13.12+13=13\left(87+12+1\right)=13.100=1300\)

2, bổ sung \(\left(x-3\right)2x+\left(x-3\right)y=\left(x-3\right)\left(2x+y\right)\)

Thay x = 13 ; y = 4 ta được : \(\left(13-3\right)\left(26+4\right)=10.30=300\)

3, \(x\left(x+4\right)-x\left(x-6\right)=x\left(x+4-x+6\right)=10x\)

Thay x = 123 ta được \(1230\)