Toán Lớp 8: a, 125x^3 – 6y^3
b, 27x^3 + y^3
c, (a+b)^3 – (a-b)^3
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Toán Lớp 8: a, 125x^3 – 6y^3
b, 27x^3 + y^3
c, (a+b)^3 – (a-b)^3
Comments ( 2 )
125x^3-8y^3
=5^3x^3-2^3y^3
=(5x)^3-(2y)^2
=(5x-2y).[(5x)^2+5x.2y+(2y)^2]
=(5x-2y)(25x^2+10xy+4y^2)
b)
27x^3+y^3
=3^3x^3+y^3
=(3x)^2+y^3
=(3x+y).[(3x)^2-3x.y+y^2]
=(3x+y)(9x^2-3xy+y^2)
c)
(a+b)^3-(a-b)^3
=(a+b-a+b).[(a+b)^2+(a+b).(a-b)+(a-b)^2]
=[(a-a)+(b+b)].(a^2+2ab+b^2+a^2-b^2+a^2-2ab+b^2)
=2b.[(a^2+a^2+a^2)+(2ab-2ab)+(b^2-b^2+b^2)]
=2b(3a^2+b^2)