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Solve the inequality | 3x + 2 < 4x + 5.

Mathematics

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1. Solve the inequality ∣3x+2∣<4x+5|3x + 2| < 4x + 5.

  1. For an inequality of the form ∣A∣<B|A| < B, we must satisfy two conditions

    A<BA < B and A>−BA > -B. Additionally, for a solution to exist, BB must be positive, so B>0B > 0.

    Condition 1: 3x+2<4x+53x + 2 < 4x + 5

    2−5<4x−3x2 - 5 < 4x - 3x −3<x-3 < x x>−3x > -3

    Condition 2: 3x+2>−(4x+5)3x + 2 > -(4x + 5)

    3x+2>−4x−53x + 2 > -4x - 5 3x+4x>−5−23x + 4x > -5 - 2 7x>−77x > -7 x>−1x > -1

    Condition 3: 4x+5>04x + 5 > 0

    4x>−54x > -5 x>−54x > -\frac{5}{4} x>−1.25x > -1.25
  2. Find the intersection of all three conditions.

    We need x>−3x > -3, x>−1x > -1, and x>−1.25x > -1.25.
    The most restrictive condition that satisfies all three is x>−1x > -1.

    The solution to the inequality is x>−1\boxed{x > -1}.

    2. Find the coefficient of x3x^3 in the expansion of (3x−2x)7(3x - \frac{2}{x})^7.

  3. Write down the general term of the binomial expansion.

    The general term Tr+1T_{r+1} in the expansion of (a+b)n(a+b)^n is given by Tr+1=(nr)an−rbrT_{r+1} = \binom{n}{r} a^{n-r} b^r.
    In this case, a=3xa = 3x, b=−2x=−2x−1b = -\frac{2}{x} = -2x^{-1}, and n=7n = 7.

    Tr+1=(7r)(3x)7−r(−2x−1)rT_{r+1} = \binom{7}{r} (3x)^{7-r} (-2x^{-1})^r
  4. Simplify the general term to isolate the powers of xx.

    Tr+1=(7r)37−rx7−r(−2)r(x−1)rT_{r+1} = \binom{7}{r} 3^{7-r} x^{7-r} (-2)^r (x^{-1})^r Tr+1=(7r)37−r(−2)rx7−rx−rT_{r+1} = \binom{7}{r} 3^{7-r} (-2)^r x^{7-r} x^{-r} Tr+1=(7r)37−r(−2)rx7−2rT_{r+1} = \binom{7}{r} 3^{7-r} (-2)^r x^{7-2r}
  5. Set the power of xx equal to 3 and solve for rr.

    We want the coefficient of x3x^3, so:

    7−2r=37 - 2r = 3 4=2r4 = 2r r=2r = 2
  6. Substitute r=2r=2 back into the coefficient part of the general term.

    The coefficient is (72)37−2(−2)2\binom{7}{2} 3^{7-2} (-2)^2.

    (72)=7!2!(7−2)!=7!2!5!=7×62×1=21\binom{7}{2} = \frac{7!}{2!(7-2)!} = \frac{7!}{2!5!} = \frac{7 \times 6}{2 \times 1} = 21 37−2=35=2433^{7-2} = 3^5 = 243 (−2)2=4(-2)^2 = 4
  7. Calculate the final coefficient.

    Coefficient=21×243×4\text{Coefficient} = 21 \times 243 \times 4 Coefficient=21×972\text{Coefficient} = 21 \times 972 Coefficient=20412\text{Coefficient} = 20412

    The coefficient of x3x^3 is 20412\boxed{20412}.

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