b) Ta có: \(B=5x\left(x-4y\right)-4y\left(y-5x\right)\)

\(=5x^2-20xy-4y^2+20xy\)

\(=5x^2-4y^2\)

\(=5\cdot\left(-\dfrac{1}{5}\right)^2-4\cdot\left(-\dfrac{1}{2}\right)^2\)

\(=5\cdot\dfrac{1}{25}-4\cdot\dfrac{1}{4}\)

\(=\dfrac{1}{5}-1=\dfrac{-4}{5}\)

a) \(A=4x^2-4x+1+9-4x^2=-4x+10\)

\(=-4.\dfrac{1}{4}+10=9\)

b) \(B=x^3+xy-x^3-8y^3=y\left(x-8y^2\right)\)

\(=\left(-2\right).\left(32-32\right)=0\)

a: Ta có: \(A=\left(2x-1\right)^2+\left(3-2x\right)\left(3+2x\right)\)

\(=4x^2-4x+1+9-4x^2\)

\(=-4x+10\)

\(=-4\cdot\dfrac{1}{4}+10=-1+10=9\)

a: \(A=x^2-2xy+y^2+x^2+2xy+y^2-2x^2-x\)

=-x

=-2

gúp mình với

A = x ( x + y ) - y ( x + y )

A = ( x + y ) ( x - y )

A = x\(^2\) - y\(^2\)

Tại x = \(\dfrac{-1}{2}\) và y = -2 ta có 

\(\left(\dfrac{-1}{2}\right)^2-\left(-2\right)^2\) \(=\) \(\dfrac{-15}{4}\)

a: Ta có: \(A=126y^3+\left(x-5y\right)\left(x^2+5xy+25y^2\right)\)

\(=126y^3+x^3-125y^3\)

\(=x^3+y^3\)

\(=\left(-5\right)^3+\left(-3\right)^3=-125-27=-152\)

b: \(C=x^3-9x^2+27x-26\)

\(=x^3-9x^2+27x-27+1\)

\(=\left(x-3\right)^3-1=\left(23-3\right)^3-1=20^3-1\)

=8000-1

=7999

c: \(D=\left(2x-3\right)^2+\left(4x-6\right)\left(4-x\right)+\left(x-4\right)^2\)

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

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

\(=\left(99+1\right)^2=100^2=10000\)

a: \(A=2\left(x+y\right)+3xy\left(x+y\right)+5x^2y^2\left(x+y\right)=0\)

b: \(B=3xy\left(x+y\right)+2x^2y\left(x+y\right)=0\)

\(\Rightarrow\)A=2(x+y)+3xy(x+y)+5x²y²(x+y)

Thay x+y=0 vào A

\(\Rightarrow\)A=0

a, Với x = 3 và y = -2 ta có:

\(A=\dfrac{3}{2}+\dfrac{4}{9}.\left(6-\left|3\right|\right)+\left(-2\right)\)

\(A=\dfrac{3}{2}+\dfrac{4}{9}.\left(6-3\right)-2\)

\(A=\dfrac{3}{2}+\dfrac{4}{9}.3-2\)

\(A=\dfrac{3}{2}+\dfrac{4}{3}-2\)

\(A=\dfrac{5}{6}\)

 Với x = 3 và y = -3 ta có:
\(B=\left|2.3-1\right|+\left|3.\left(-3\right)+2\right|\)

\(B=\left|5\right|+\left|-7\right|\)

\(B=5+7=12\)

Hoctot ! ko hiểu chỗ nào cứ hỏi cj nhé

E cảm ơn cj

Ta có:

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

Thay \(x=-3\) vào A, ta có:

\(A=2.\left(-3\right)^2=18\)

Vậy A=18

\(A=x\left(x+y\right)-x\left(y-x\right)=x\left(x+y\right)+x\left(x+y\right)=\left(x+y\right).2x=\left(-3+2\right).2.\left(-3\right)=6\)