Algebra · real student question

Simplify (x + 1)(x + 1)(x + 1)(x + 1) - (x + 3)(x + 3)(x + 3)(x + 3) - 272.

Question

Simplify

(x+1)4(x+3)4272(x+1)^4-(x+3)^4-272

Step-by-step solution

  1. Rewrite the repeated products as fourth powers. The four identical factors in each group are just powers:

    (x+1)(x+1)(x+1)(x+1)=(x+1)4,(x+3)(x+3)(x+3)(x+3)=(x+3)4(x+1)(x+1)(x+1)(x+1)=(x+1)^4,\qquad (x+3)(x+3)(x+3)(x+3)=(x+3)^4

  2. Expand each with the binomial theorem. Using coefficients 1,4,6,4,11,4,6,4,1:

    (x+1)4=x4+4x3+6x2+4x+1(x+1)^4=x^4+4x^3+6x^2+4x+1

    (x+3)4=x4+4(3)x3+6(9)x2+4(27)x+81=x4+12x3+54x2+108x+81(x+3)^4=x^4+4(3)x^3+6(9)x^2+4(27)x+81=x^4+12x^3+54x^2+108x+81

    The powers of 33 grow quickly — 3,9,27,813,9,27,81 — and that is where arithmetic slips happen.

  3. Subtract term by term, distributing the minus over the whole second expansion.

    x4x4=0,4x312x3=8x3,6x254x2=48x2,4x108x=104xx^4-x^4=0,\quad 4x^3-12x^3=-8x^3,\quad 6x^2-54x^2=-48x^2,\quad 4x-108x=-104x

    The leading x4x^4 terms always cancel when two monic fourth powers are subtracted, so the result drops to degree 33.

  4. Handle the constants, including the 272-272.

    181272=3521-81-272=-352

    so the simplified expression is

    8x348x2104x352-8x^3-48x^2-104x-352

  5. Factor out the common 8-8 and verify. Each coefficient is divisible by 88:

    8(x3+6x2+13x+44)-8\left(x^3+6x^2+13x+44\right)

    Checking at x=0x=0: the original is 181272=3521-81-272=-352 and the simplified form gives 352  -352\;\checkmark. At x=1x=1: the original is 16256272=51216-256-272=-512, and 848104352=512  -8-48-104-352=-512\;\checkmark. The cubic factor has no rational roots among the divisors of 4444, so this is as far as integer factoring goes.

Answer

8x348x2104x352=8(x3+6x2+13x+44)-8x^3-48x^2-104x-352=-8\left(x^3+6x^2+13x+44\right)

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