a) Ta có:

   (2x²-3xy+y²)(x+y)

= x.(2x²-3xy+y²)+y.(2x²-3xy+y²)

= 2x³-3x²y+xy²+2x²y-3xy²+y³

= 2x³-x²y-2xy²+y³

^{b) Ta có:}

   (x-2)(x²-5x+1)-x(x²+11)

= x.(x²-5x+1)-2.(x²-5x+1)-x(x²+11)

= x³-5x²+x-2x²+10x-2-x³-11x

= -7x²-2

c) Ta có:

   [(x²-2xy+2y²)(x+2y)-(x²+4y²)(x-y)]2xy

= [x.(x²-2xy+2y²)+2y.(x²-2xy+2y²)-x.(x²+4y²)+y.(x²+4y²)].2xy

= (x³-2x²y+2xy²+2x²y-4xy²+4y³-x³-4xy²+x²y+4y³).2xy

= (x²y-6xy²+8y³).2xy

= 2x³y²-12x²y³+16xy⁴

chứng minh số sau là số chính phương: b=111...1555...56 ( n số 1, n-1 số 5)