Question

cho x - 2y = 8.
Tính giá trị biểu thức: A=x(x+2y-5)+y(4y-5x+10)-xy

Answer 1

 A=x(x+2y-5)+y(4y-5x+10)-xy

<=> \(A=x^2+2xy-5x+4y^2-5xy+10y-xy\)

<=> \(A=x^2-4xy-5x+4y^2+10y\)

<=> \(A=\left(x^2-4xy+4y^2\right)-5\left(x-2y\right)\)

<=> \(A=\left(x-2y\right)^2-5\left(x-2y\right)\)

Thay x - 2y = 8 vào A ta được 

A = 8² - 5.8

<=> A = 64 - 40 = 24

Answer 2

ta có \(x=8+2y\)

\(A=\left(8+2y\right)\left(8+2y+2y-5\right)+y\left(4y-40-10y+10\right)-y\left(8+2y\right)\)

\(=\left(8+2y\right)\left(3+4y\right)-y\left(30+6y\right)-y\left(8+2y\right)\)

\(=\left(3+4y-y\right)\left(8+2y\right)-30y-6y^2=24+6y^2+24y+6y-30y-6y^2=24\)