Tìm x biết

16 .thuc hien phep tinh
a. ( 5x - 2y)(x^2 - xy + 1 )
b. ( x - 1 )( x + 1 )( x + 2 )
c.1/2 x^2 y^2 ( 2x + y ) ( 2x - y )

17.chung minh
a. ( x-1 ) ( x^2 + x +1 ) = x^3 - 1 
b. ( x^3 + x^2 y + xy^2 + y^3 )( x - y ) = x^4 - y^4

20. tim x
a. 4x ( x -5 ) - ( x -1)( 4x -3 ) = 5 
b. ( x - 5 ) ( x - 1 ) = ( x - 1 ) ( x - 2 ) 
c. ( x - 5 ) ( - x + 4 ) - ( x - 1 ) ( x + 3 ) = - 2x^2 
d. ( x + 1 ) ( x^2 - x + 1 ) - x ( x^2 - 3 ) = 4