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Find the binomial expansion of x2(x-2y)3 and the term independent of x in the expansion of (x - (3)/(x))4.

Mathematics

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1. Find the binomial expansion of x2(x−2y)3x^2(x-2y)^3.

  1. Expand (x−2y)3(x-2y)^3.

    (x−2y)3=x3+3x2(−2y)+3x(−2y)2+(−2y)3(x-2y)^3 = x^3 + 3x^2(-2y) + 3x(-2y)^2 + (-2y)^3 =x3−6x2y+12xy2−8y3= x^3 - 6x^2y + 12xy^2 - 8y^3
  2. Multiply by x2x^2.

    x2(x3−6x2y+12xy2−8y3)=x5−6x4y+12x3y2−8x2y3x^2(x^3 - 6x^2y + 12xy^2 - 8y^3) = x^5 - 6x^4y + 12x^3y^2 - 8x^2y^3

    The final answer is x5−6x4y+12x3y2−8x2y3\boxed{x^5 - 6x^4y + 12x^3y^2 - 8x^2y^3}.

    2. Find the term independent of xx in the expansion of (x−3x)4\left(x - \frac{3}{x}\right)^4.

  3. General term Tk+1T_{k+1}.

    Tk+1=(4k)(x)4−k(−3x)k=(4k)(−3)kx4−kx−k=(4k)(−3)kx4−2kT_{k+1} = \binom{4}{k} (x)^{4-k} \left(-\frac{3}{x}\right)^k = \binom{4}{k} (-3)^k x^{4-k} x^{-k} = \binom{4}{k} (-3)^k x^{4-2k}
  4. For term independent of xx, set 4−2k=04-2k=0.

    4−2k=0  ⟹  k=24-2k = 0 \implies k=2
  5. Substitute k=2k=2.

    T2+1=(42)(−3)2x4−2(2)=6⋅9⋅x0=54T_{2+1} = \binom{4}{2} (-3)^2 x^{4-2(2)} = 6 \cdot 9 \cdot x^0 = 54

    The final answer is 54\boxed{54}.

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