1. N = x6 -20x5+20x4-20x3+20x2-20x+20 biết x=19
thay x+1 = 20
=> x6-(x+1)x5+(x+1)x4-( x+1)x3+(x+1)x2-(x+1)x+20
= x6-x6-x5+x5+x4-x4-x3+x3+x2-x2-x+20
=-x+20
thay x=19, thì N= -19+20=10
Vậy N=1 tại x=19
thay x+1 = 20
=> x6-(x+1)x5+(x+1)x4-( x+1)x3+(x+1)x2-(x+1)x+20
= x6-x6-x5+x5+x4-x4-x3+x3+x2-x2-x+20
=-x+20
thay x=19, thì N= -19+20=10
Vậy N=1 tại x=19