(x^3+27)-x(x^2-4)=15

x^3+27-x^3-4x=15

-4x=-12

x=3

Hình như bạn ghi sai đề. Đề đúng: \(\left(5x+1\right)^2-\left(5x-3\right)\left(5x+3\right)=30\)

\(\left(5x+1\right)^2-\left(5x-3\right)\left(5x+3\right)=30\)

\(\Leftrightarrow25x^2+10x+1-25x^2+9=30\)

\(\Leftrightarrow10x+10=30\)

\(\Leftrightarrow10x=20\)

\(\Leftrightarrow x=2\)

câu 6 bn chép thiếu 

caau7: p = 10100/2

           p = 2p - p = 5050

ID=1/4BD đó

\(\left(x+y\right)=3\Leftrightarrow\left(x+y\right)^2=9\Leftrightarrow x^2+y^2+2xy=9\Leftrightarrow5+2xy=9\Leftrightarrow xy=2.\)

\(x^3+y^3=\left(x+y\right)\left(x^2+y^2-xy\right)=3.\left(5-2\right)=9\)

Câu 6:

\(\left(x-2016\right)^2\ge0\) với mọi x

\(\left(x+2017\right)^2\ge0\) với mọi y

\(\Rightarrow\left(x-2016\right)^2+\left(y+2017\right)^2=0\) Khi \(\left(x-2016\right)^2=0\Leftrightarrow x=2016\) và \(\left(x+2017\right)^2=0\Leftrightarrow x=-2017\)

\(\Rightarrow x+y=2016-2017=-1\)

Câu 7:

 \(D=\left(x+y\right)^2-6\left(x+y\right)-15=\left(-9\right)^2-6.\left(-9\right)-15=120\)

\(Q=\left(x+y\right)^2-4\left(x+y\right)+1=3^2-4.3+1=-2\)

câu 5:

x²+y²=5   -> x²+2xy+ y²-2xy=5

                -> (x+y)² - 2xy = 5 -> 3²  - 2xy = 5 ->xy = 2

có x³+ y³= (x+y).(x²-xy+y²)

              = 3.( 5- 2)= 9

vậy x³+ y³ =9

câu 6:

( x - 2016)²  ≥ 0 dấu = xảy ra khi x=2016

 ( y + 2017 )²  ≥ 0 dấu bằng xảy ra khi y = 2016

-> ( x - 2016)² + ( y + 2017 )²  ≥ 0 dấu bằng xảy ra khi x=2016, y = 2017

-> x+y=2016+2017=4033

câu 7:

a,

D = x² +2xy +y²  - 6x - 6y  -15= (x² +2xy +y²⁾  - (6x + 6y)  -15= (x+y)² - 6(x+y) - 15

D= (-9)² -6.(-9)-15=120

b,

Q = x² + 2xy + y²  - 4x - 4y +1 = (x² + 2xy + y²⁾  - (4x + 4y) +1

Q= (x+y)²-4.(x+y)+1

Q=3²- 4.3 +1= -2