1) Cho x+y=2 và x^2+y^2=10. Tính x^3+y^3. Giải

(x+y)^2=x^2+y^2+2xy => xy= -3 
x^3+y^3=(x+y)^3-3xy(x+y) = 26

2) Ta có: x^3+y^3 = (x+y)(x^2-xy+y^2) (1)

(x+y)^2=a^2

=> x^2 +2xy +y^2=a^2

=> b+2xy=a^2

=> xy=\(\frac{a^2-b}{2}\)

Thay (1) vào đó ta có:

x^3+y^3= (x+y)(x^2-xy+y^2) = a(b-\(\frac{a^2-b}{2}\)) = \(a\left(\frac{2b-a^2+b}{2}\right)=a.\frac{3b-a^2}{2}\)

\(x^3+y^3=\left(x+y\right)\left(x^2-xy+y^2\right)=2\left(10-xy\right)\)

Ta có: \(x^2+y^2=\left(x+y\right)^2-2xy=2^2-2xy=4-2xy=10\Rightarrow2xy=-6\Rightarrow xy=-3\)

Vậy: \(x^3+y^3=2\left(10+3\right)=2.13=26\)

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

a) \(\left(x+y\right)^2=x^2+y^2+2xy\Rightarrow4=10+2xy\Leftrightarrow xy=-3\)

\(x^3+y^3=\left(x+y\right)^3-3xy\left(x+y\right)=2^3+3.3.2=26\)

b) \(\left(x-y\right)^2=x^2+y^2-2xy\Rightarrow m^2=n-2xy\Leftrightarrow xy=\frac{n-m^2}{2}\)

\(x^3-y^3=\left(x-y\right)^3+3xy\left(x-y\right)=m^3+3.m.\frac{n-m^2}{2}=\frac{3mn}{2}-\frac{m^3}{2}\)

x+y=a=>(x+y)²=a²=>x²+2xy+y²=a²=>b+2xy=a²=>xy=(a²-b)/2

x³ + y³ = ( x + y ) ( x² - xy + y² ) = a[ b - (a²-b)/2 ] = ( 3ab - a² )/2.

Dau bang cuoi cung la: (3ab-a³)/2.

\(A=x^3+y^3+3xy=\left(x+y\right)^3-3xy\left(x+y\right)+3xy=1+0=1\)

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

\(c,M=a^2-ab+b^2+3ab\left(a^2+b^2\right)+6a^2b^2=3ab\left(a^2+2ab+b^2\right)+a^2-ab+b^2\)

\(=3ab+a^2-ab+b^2=\left(a+b\right)^2=1\)

\(x+y=2;x^2+y^2=10\text{ do đó:}xy=-3\text{ nên }\left(x-y\right)^2=16\text{ do đó: }x-y=4\text{ hoặc }x-y=-4\)

\(\text{giải ra được:}x=3;y=-1\text{ hoặc ngược lại nên }x^3+y^3=-26\text{ hoặc }26\)

A = x³ + y³ + 3xy

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

= ( x³ + 3x² + 3xy² + y³ ) - ( 3x²y + 3xy - 3xy )

= ( x + y )³ - 3xy( x + y - 1 )

= 1³ - 3xy( 1 - 1 )

= 1³ - 3xy.0

= 1 - 0 = 1

Vậy A = 1

b) B = x³ - y³ - 3xy

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

= ( x³ - 3x²y + 3xy² - y³ ) + ( 3x²y - 3xy² - 3xy )

= ( x - y )³ + 3xy( x - y - 1 )

= 1³ + 3xy( 1 - 1 )

= 1 + 3xy.0

= 1 + 0 = 1

Vậy B = 1

M = a³ + b³ + 3ab( a² + b² ) + 6a²b²( a + b )

= ( a + b )( a² - ab + b² ) + 3ab[ ( a + b )² - 2ab ] + 6a²b²( a + b )

= ( a + b )[ ( a + b )² - 3ab ] + 3ab[ ( a + b )² - 2ab ] + 6a²b²( a + b )

= 1.( 1 - 3ab ) + 3ab( 1 - 2ab ) + 6a²b².1

= 1 - 3ab + 3ab - 6a²b² + 6a²b²

= 1

Vậy M = 1

d) x + y = 2

⇔ ( x + y )² = 4

⇔ x² + 2xy + y² = 4

⇔ 10 + 2xy = 4 ( gt x² + y² = 10 )

⇔ 2xy = -6

⇔ xy = -3

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

            = ( x³ + 3x²y + 3xy² + y³ ) - ( 3x²y + 3xy² )

            = ( x + y )³ - 3xy( x + y )

            = 2³ - 3.(-3).(2)

            = 8 + 18 = 26

d đâu bạn

Các bài này đưa về dạng Hằng đẳng thức là được . Làm ra dài lắm bạn ạ !

Ns nghe dễ lắm, lm thử đee =))

A. \(x+y=1\Rightarrow\left(x+y\right)^3=1\Rightarrow x^3+3x^2y+3xy^2+y^3=1\)

\(\Rightarrow x^3+3xy\cdot\left(x+y\right)+y^3=1\)

\(\Rightarrow x^3+3xy+y^3=1\)

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

\(\Rightarrow x^3-3xy\cdot\left(x-y\right)-y^3=1\)

\(\Rightarrow x^3-3xy-y^3=1\)

C. \(M=a^3+b^3+3ab\left[\left(a+b\right)^2-2ab\right]+6a^2b^2\left(a+b\right)\)

\(M=a^3+b^3+3ab\left(1-2ab\right)+6a^2b^2=a^3+b^3+3ab-6a^2b^2+6a^2b^2\)

\(M=a^3+b^3+3ab=1\)(Theo hệ quả câu A).

D. Từ \(x+y=2\Rightarrow\left(x+y\right)^2=4\Rightarrow x^2+y^2+2xy=4\Rightarrow10+2xy=4\Rightarrow xy=-3\)

Mà, \(\left(x+y\right)^3=x^3+y^3+3xy\left(x+y\right)\)

\(\Leftrightarrow2^3=x^3+y^3+3\left(-2\right)\cdot2\Leftrightarrow x^3+y^3=8+12=20\)

Ta có : \(A=x^2+y^2=x^2+2xy+y^2-2xy\)

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

Với \(x+y=3\) và \(xy=-10\)

\(\Rightarrow A=3^2-2.\left(-10\right)\)

\(A=9+20\)

\(A=29\)

Tương tự : \(B=x^3+y^3=\left(x+y\right)^3-3xy.\left(x+y\right)\)

\(B=\left(3\right)^3-3.\left(-10\right).3\)

\(B=117\)