Ta có: 4B=6C => B=6C/4=3C/2

B + C = 180 độ (AB//CD; trong cùng phía)

=> 3C/2 + C = 180 độ

=>5C/2 = 180 độ

=>C = 40

=>B=180-40=140

Có: A+B+C+D=360 độ => góc D = 80

\(x^2=a^2+b^2+ab\)

\(\Leftrightarrow x^4=a^4+b^4+a^2b^2+2a^2b^2+2ab^3+2a^3b\)

\(\Leftrightarrow2x^4=2a^4+2b^4+6a^2b^2+4ab^3+4a^3b\)

\(\Leftrightarrow2x^4=a^4+b^4+\left(a^2\right)^2+\left(b^2\right)^2+\left(2ab\right)^2+2a^2b^2+2b^2.2ab+2.2ab.a^2\)

\(\Leftrightarrow2x^4=a^4+b^4+\left(a^2+b^2+2ab\right)^2\)

\(\Leftrightarrow2x^4=a^4+b^4+\left[\left(a+b\right)^2\right]^2\)

\(\Leftrightarrow2x^4=a^4+b^4+c^4\left(đpcm\right)\)

(x-6)²-x²=x-1

<=> (x-6-x)(x-6+x)=x-1

<=> -6(2x-6)=x-1

<=> -12x+36=x-1

<=> 13x=35

<=> x=\(\frac{35}{13}\)

Câu kia bn xem lại đề nhé

Chúc hok tốt

Thank you Trần Thùy Linh. Giúp mình câu này nhé:

( x - 3 ) nhân ( x - 2 ) = ( x - 4 )

(x + y)³ - 1 - 3xy(x + y - 1)

= x³ + 3x²y + 3xy² + y³ - 1 - 3x²y - 3xy² + 3xy

= x³ - 1 + 3xy

= x(x² + 3y) - 1

k bt lm nx r :v

\(\left(x+y\right)^3-1-3xy\left(x+y-1\right) \)

\(=\left(x+y-1\right)\left[\left(x+y\right)^2+x+y+1\right]-3xy\left(x+y-1\right)\)

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

\(=\left(x+y-1\right)\left(x^2-xy+y^2+x+y+1\right)\)

Đặt  \(A=x^2+y^2+xy+3x+3y+2018\)

\(4.A=4x^2+4y^2+4xy+12x+12y+8072\)

\(4.A=\left(4x^2+4xy+y^2\right)+3y^2+12x+12y+8072\)

\(4.A=\left[\left(2x+y\right)^2+2\left(2x+y\right).3+9\right]+3\left(y^2+2y+1\right)+8060\)

\(4.A=\left(2x+y+3\right)^2+3\left(y+1\right)^2+8060\)

Mà  \(\left(2x+y+3\right)^2\ge0\forall x;y\)

       \(\left(y+1\right)^2\ge0\forall y\)\(\Rightarrow3\left(y+1\right)^2\ge0\forall y\)

\(\Rightarrow4.A\ge8060\)

\(\Leftrightarrow A\ge2015\)

Dấu "=" xảy ra khi : 

\(\hept{\begin{cases}2x+y+3=0\\y+1=0\end{cases}}\Leftrightarrow\hept{\begin{cases}x=-1\\y=-1\end{cases}}\)

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