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Algebra · real student question

Solve the inequality -2|3m| + 3 < 51.

Question

Solve

−2∣3m∣+3<51-2|3m|+3<51

Step-by-step solution

  1. Isolate the absolute value term. Subtract 33 from both sides:

    −2∣3m∣<48-2|3m|<48

  2. Divide by −2-2 and reverse the sign. Division by a negative flips the direction:

    ∣3m∣>−24|3m|>-24

  3. Stop and read the inequality instead of splitting it. The usual split A<−bA<-b or A>bA>b assumes b>0b>0. Here the right-hand side is negative, so that template does not apply. Compare the two sides directly:

    ∣3m∣≥0>−24|3m|\ge 0>-24

  4. Conclude that every real number is a solution. Since ∣3m∣|3m| is at least 00 and 00 already exceeds −24-24, the inequality holds for every mm:

    m∈(−∞,∞)m\in(-\infty,\infty)

    Spot-check m=0m=0: −2⋅0+3=3<51-2\cdot 0+3=3<51 ✓\checkmark; and m=1000m=1000: −6000+3=−5997<51-6000+3=-5997<51 ✓\checkmark.

  5. Compare with the near-identical problem that is not trivial. Changing the right-hand side from 5151 to −51-51 gives ∣3m∣>27|3m|>27, whose solution is only m<−9m<-9 or m>9m>9. The sign of the number left after the flip is what decides between 'all reals' and a genuine two-branch answer.

Answer

m∈(−∞,∞)(every real number)m\in(-\infty,\infty)\quad\text{(every real number)}

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