a)B=(3x.4x-11)+5x2.(x-1)-4x.(3x+9)+x.5x-5x2
=12x2-11+5x3-5x2-12x2-36x+5x2-5x2
=5x3+(12x2-12x2+5x2-5x2-5x2)-36x-11
=5x3-5x2-36x-11
b)|x|=2=>x=2 hoặc x=-2
*)x=2 =>B=5.23-5.22-36.2-11=40-20-72-11=-63
*)x=-2 =>B=5.(-2)3-5.(-2)2-36.(-2)-11=-40-20+72-11=1
c)B=207
=>5x3-5x2-36x-11=207
<=>5x3-5x2-36x-218=0(bó tay)