Hằng đẳng thức dạng ( A – B )^3 = A^3 -3A^2B + 3AB^2 – B^3 a) – ( 4 – 2y )^3 b) – ( a – 2 )^3 c) ( 2x – 3y )^3 d) ( x -1/3 y)^3 giúp mik nha mik đa

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1. Đáp án:

   a)$ – (4 – 2y)^{3}$

   $= – [4^3 – 3 . 4^{2} . 2y + 3 . 4 . (2y)^{2} – (2y)^{3}]$

   $= – (64 – 96y + 48y^{2} – 8y^{3})$

   $= -64 + 96y – 48y^{2} + 8y^{3}$

   b)$ – (a – 2)^{3}$

   $= – (a^{3} – 3 . a^{2} . 2 + 3 . a . 2^{2} – 2^{3})$

   $= – (a^{3} – 6a^{2} + 12a – 8)$

   $= -a^{3} + 6a^{2} – 12a + 8$

   c) $(2x – 3y)^{3}$

   $= (2x)^{3} – 3 . (2x)^{2} . 3y + 3 . 2x . (3y)^{2} – (3y)^{3}$

   $= 8x^{3} – 36x^{2}y + 54xy^{2} – 27y^{3}$

   d)$ (x -$\dfrac{1}{3}$ y)^{3}$

   $= x^{3} – 3 . x^{2} . \dfrac{1}{3}y + 3 . x . (\dfrac{1}{3}y)^{2} – (\dfrac{1}{3}y)^3$

   $= x^{3} – x^{2}y + \dfrac{1}{3}xy^{2} – \dfrac{1}{27}y^{3}$

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2. a) – (4 – 2y)^3

   = – [4^3 – 3.4^2.2y + 3.4.(2y)^2 – (2y)^3]

   = – (64 – 96y + 48y^2 – 8y^3)

   = -64 + 96y – 48y^2 + 8y^3

   b) – (a – 2)^3

   = – (a^3 – 3.a^2.2 + 3.a.2^2 – 2^3)

   = – (a^3 – 6a^2 + 12a – 8)

   = -a^3 + 6a^2 – 12a + 8

   c) (2x – 3y)^3

   = (2x)^3 – 3.(2x)^2.3y + 3.2x.(3y)^2 – (3y)^3

   = 8x^3 – 36x^2y + 54xy^2 – 27y^3

   d) (x -1/3y)^3

   = x^3 – 3.x^2.1/3y + 3.x.(1/3y)^2 – (1/3y)^3

   = x^3 – x^2y + 1/3xy^2 – 1/27y^3

   HỌC TỐT ^^

   @vietdorapan

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