x + y = -5 => x = -5 - y

xy = 6 <=> ( -5 - y )y = 6

<=> -y² - 5y = 6

<=> y² + 5y + 6 = 0

<=> ( y + 2 )( y + 3 ) = 0

<=> y = -2 hoặc y = -3

Với y = -2 => x = -3

Với y = -3 => x = -2

Vậy ( x ; y ) = { ( -2 ; - 3 ) , ( -3 ; -2 ) }

Ta thấy :

xy = 6 

=> xy \(\in\)\(\left\{3.2;\left(-3\right).\left(-2\right);1.6;\left(-1\right).\left(-6\right)\right\}\)

Mà x + y = -5

Xét 

xy = 3.2 => x + y = 5 ( loại )

xy = ( -3 ) . ( -2 ) => x + y = -5 ( TM )

xy = 1 . 6 => x + y = 7 ( loại )

xy = ( -1 ) . ( -6 ) => x + y = -7 ( loại )

Vậy x = -3 ; y = -2 hoặc x = -2 ; y = -3

\(\hept{\begin{cases}x+y=-5\\xy=6\end{cases}}\Leftrightarrow\hept{\begin{cases}y=-5-x\\x.\left(-5-x\right)=6\end{cases}}\Leftrightarrow\hept{\begin{cases}y=-5-x\\x^2+5x+6=0\end{cases}}\)

\(\Leftrightarrow\hept{\begin{cases}y=-5-x\\\left(x+2\right)\left(x+3\right)=0\end{cases}}\Leftrightarrow\orbr{\begin{cases}x=-2,y=-3\\x=-3,y=-2\end{cases}}\)

Vì xy = 6

=> \(x=\frac{6}{y}\)

Khi đó x + y = -5

<=> \(\frac{6}{y}+y=-5\)

=> \(\frac{y^2+6}{y}=-5\)

=> y² + 6 = -5y

=> y² + 5y + 6 = 0

=> y² + 2y + 3y + 6 = 0

=> y(y + 2) + 3(y + 2) = 0

=> (y + 3)(y + 2) = 0 

=> \(\orbr{\begin{cases}y+3=0\\y+2=0\end{cases}}\Rightarrow\orbr{\begin{cases}y=-3\\y=-2\end{cases}}\)

Khi y = -3 => x = -2

Khi y = -2 => x = -3

Vậy các cặp (x;y) thỏa mãn là (-2 ; - 3) ; (-3 ; -2)

(a^4+b^4+c^4)^3.(1+1+1)≥(a^3+b^3+c^3)^4≥(a^3+b^3+c^3)3.\(\frac{\text{(a+b+c)^3 }}{9}\)

=3(a^3+b^3+c^3)^3
⇒a^4+b^4+c^4≥a^3+b^3+c^3

2) She runs faster than me.

I don't run faster than her

3) Thanh works harder now than last year.

Last year Thanh didn't work harder than now

4) He doesn't understand the lesson as well as last year.

Last year he he didn't understand the lesson well

5) My sister usually gets up earlier than me .

I usually wake up later than my sister

2) She runs faster than me.

I don't run fasster than her

3) Thanh works harder now than last year.

Last year Thanh didn't work harder than now

4) He doesn't understand the lesson as well as last year.

Last year he understand the lesson as well as now

5) My sister usually gets up earlier than me .

I usually get up later than my sister

bài 1 rewrite sentences

1) Hai couldn't listen to the teacher yesterday as attentively as his friend did.

 Hai's friend listened more attentively to the teacher lecture

2) She runs faster than me.

I don't run faster than her

Who ...(stand) behind the curtains last night?

Trả lời :

Who was standing behind the curtain last night?

#ht

who _was_ stood_ behind the curtains last night?