Question

Cho x+y=3 . Tính giá tr biểu thức : A=x³+x²y-3x²+xy+y²-4y-x+3

Answer 1

A = x³ + x²y - 3x² + xy + y² - 4y - x + 3

= x³ + x²y + xy + y² - x - y - 3x² - 3y + 3

= ( x + y ) ( x² + y - 1 ) - 3 ( x² + y - 1 )

= ( x² + y - 1 ) ( x + y - 3 )

Thay x + y = 3 vào A, ta có :

A = ( x² + y - 1 ) ( 3 - 3 ) = 0

Answer 2

Ta có: x + y = 3.

=> y = 3 - x

A = x³ + x²y - 3x² + xy + y² - 4y - x + 3

A = x²(x + y) - 3(x² - 1) + y(x + y) -(x + y) - 3(y - 1)

A = 3x² - 3(x² - 1) + 3y - 3

A = 3(x² - x² + 1 + y - 1)

A = 3y = 3(3 - x) ??????????????????????