26. a) (2x2 + 3y)3
=(2x2
)3 + 3.(2x2)2.3y+3.2x2.(3y)2+(3y)3
=8x6+3.4x4.3y+3.2x2.9y2+27y3
=8x6+394x4y+18x2y2+27y3
27. a) –x3+3x2-3x+1
=1-3x+3x
=(1-x)32-x3
b) 8-12x+6x2-x3
=23-3.22.1x+2.3.1x2-x3
=(2-x)3
28. a) x3+12x2+48x+64
=x3+3.(1x)2.4+3.1x.42+43
=(x+4)3
Ta có tại x=6
=(6+4)3
=103=1000
b) x3-6x2+12x-8
= x3-3.(1x)2.2+3.1x.22-23
=(x-2)3
Ta có tại
x=22
=(22-2)3
=203=800030. a) (x+3)(x2-3x+9)-(54+x3)
=203=800030. a) (x+3)(x2-3x+9)-(54+x3)
=(x+3)(x2-3x+32)-(54+x3)
=(x3+3)-(54+x3)
=x3+3-54+x3
=2x3-51
b) (2x+y)(4x2-2xy+y2)-(2x-y)(4x2+2xy+y2)
=(2x)3+y3-(2x)3-y3
=0
31. a) a3+b3=(a+b)3-3ab(a+b)
=a3+3a2b+3ab2+b3-3a2b-3ab2
=a3+b3
b) a3-b3=(a-b)3+3ab(a-b)
=a3-3a2b+3ab2-b3+3a2b-3ab2
=a3-b3
Ta có
a.b=6 và a+b=-5
a) (a+b)3-3ab(a+b)
= (-5)3-3.6.(-5)
= -125+90
= -35
32. a) (3x+y)(3x)2-3xy+y2)=27x3+y3
b)
(2x-5)(4x2+10x+25)=8x3-125
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